drm/radeon/kms: improve bpc handling (v2)

Improve handling of bpc (bits per color) in radeon.
In most cases we want 8 except for HDMI, DP, LVDS, and eDP.

v2: handle DP better.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>