MLK-18576-3 drm/imx: ldb: Add dual channel mode support for i.MX8dx/dxp/qxp

i.MX8dx/dxp/qxp use two LDBs(one primary, one auxiliary) to support
dual channel mode.  This patch adds the dual channel mode support
for i.MX8dx/dxp/qxp.  Note that the drivers contain specific sequence
needed by this mode - LDB VSYNC polarity and channel selection settings
should be configured into the register a bit earlier in ->atomic_mode_set
instead of in ->enable, and DC subsystem pixel link enablement is moved
from the DPU driver to the LDB driver to make sure it happens later
than LDB clocks enablement in ->enable.

Signed-off-by: Liu Ying <victor.liu@nxp.com>