drm/radeon: unify bpc handling

drm/radeon: unify bpc handling

We were already storing the bpc (bits per color) information
in radeon_crtc, so just use that everywhere rather than
calculating it everywhere we use it.  This also allows us
to change it in one place if we ever want to override it.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>