drm/amdgpu: Remove in_interrupt() usage in gfx_v9_0_kiq_read_clock()

gfx_v9_0_get_gpu_clock_counter() acquires a mutex_t lock and is the only
caller of gfx_v9_0_kiq_read_clock().
If it safe to acquire a mutex_t then gfx_v9_0_get_gpu_clock_counter() is
always invoked from preemptible context.

Remove in_interrupt() because it superfluous as it will always return
false.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>