drm/radeon: use pixel formats instead of depth/bpp

This disambiguates depth 16 formats, such as ARGB1555 and ARGB4444,
and depth 32 formats such as ARGB2101010 and ARGB8888.

This patch also adds support for depth 30 (XRGB2101010) framebuffers.

Signed-off-by: Fredrik Höglund <fredrik@kde.org>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>