drm: Unify radeon offset checking.

Replace r300_check_offset() with generic radeon_check_offset(), which doesn't
reject valid offsets when the framebuffer area is at the very end of the card's
32 bit address space. Make radeon_check_and_fixup_offset() use
radeon_check_offset() as well.

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=7697 .