MLK-21383 drm/bridge: sec-dsim: fix maybe-uninitialized build warnings

If enable CONFIG_DYNAMIC_DEBUG, below maybe-uninitialized build
warnings happen:

"
In file included from ./include/linux/printk.h:336:0,
                 from ./include/linux/kernel.h:14,
                 from ./include/linux/unaligned/access_ok.h:5,
                 from ./include/asm-generic/unaligned.h:13,
                 from ./arch/arm64/include/generated/asm/unaligned.h:1,
                 from drivers/gpu/drm/bridge/sec-dsim.c:17:
drivers/gpu/drm/bridge/sec-dsim.c: In function ‘sec_mipi_dsim_calc_pmsk’:
./include/linux/dynamic_debug.h:135:3: warning: ‘best_s’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   __dynamic_dev_dbg(&descriptor, dev, fmt, \
   ^
drivers/gpu/drm/bridge/sec-dsim.c:1149:27: note: ‘best_s’ was declared here
  uint32_t best_p, best_m, best_s;
                           ^
In file included from ./include/linux/printk.h:336:0,
                 from ./include/linux/kernel.h:14,
                 from ./include/linux/unaligned/access_ok.h:5,
                 from ./include/asm-generic/unaligned.h:13,
                 from ./arch/arm64/include/generated/asm/unaligned.h:1,
                 from drivers/gpu/drm/bridge/sec-dsim.c:17:
./include/linux/dynamic_debug.h:135:3: warning: ‘best_m’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   __dynamic_dev_dbg(&descriptor, dev, fmt, \
   ^
drivers/gpu/drm/bridge/sec-dsim.c:1149:19: note: ‘best_m’ was declared here
  uint32_t best_p, best_m, best_s;
                   ^
In file included from ./include/linux/printk.h:336:0,
                 from ./include/linux/kernel.h:14,
                 from ./include/linux/unaligned/access_ok.h:5,
                 from ./include/asm-generic/unaligned.h:13,
                 from ./arch/arm64/include/generated/asm/unaligned.h:1,
                 from drivers/gpu/drm/bridge/sec-dsim.c:17:
./include/linux/dynamic_debug.h:135:3: warning: ‘best_p’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   __dynamic_dev_dbg(&descriptor, dev, fmt, \
   ^
drivers/gpu/drm/bridge/sec-dsim.c:1149:11: note: ‘best_p’ was declared here
  uint32_t best_p, best_m, best_s;
           ^
"

Although this is a spurious warning according to the code logic, it
is better to give 'best_p', 'best_m' and 'best_s' initial values to
suppress the warnings.

Reported-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Fancy Fang <chen.fang@nxp.com>