drm/msm/dp: use correct bpp in validate_mode

Adjust the implementation to use the correct bpp based on
dsc/no-dsc mode in validate_mode function.

CRs-Fixed: 2325207
Change-Id: I05dc619cff15380290dc5e113bf0cb04d669a082
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>