drm/radeon/kms: Update memory bandwidth requirements in mode_set_base hook.

The hook may change the number of bytes per pixel being scanned out, which
affects the CRTC memory bandwidth requirements. E.g. booting in 8bpp and then
running X in 32bpp would result in the bandwidth requirements being
underestimated for the latter and consequently in CRTC FIFO underruns causing
visible artifacts with 3D intensive workloads.

ATOM changes only compile-tested.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>