drm/amd/display: Use u16 for drm_bpp in DSC calculations

[Why]
DSC calculations fail because the u16 bits_per_pixel from
the DRM struct is being casted to the u8 drm_bpp parameters
and locals. Integer wraparound is happening because this
value is greater than 255.

[How]
Use u16 to match what's in the structure instead of u8.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>