drm/radeon/kms: fix and unify tiled buffer alignment checking for r6xx/7xx

Tiled buffers have the same alignment requirements regardless of
whether the surface is for db, cb, or textures.  Previously, the
calculations where inconsistent for each buffer type.

- Unify the alignment calculations in a common function
- Standardize the alignment units (pixels for pitch/height/depth,
bytes for base)
- properly check the buffer base alignments

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>