drm/amd/display: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./drivers/gpu/drm/amd/display/dc/dml/dsc/rc_calc_fpu.c:96:14-15: WARNING
comparing pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>