freedreno: move UBWC color offset to fd_resource_offset()

freedreno: move UBWC color offset to fd_resource_offset()

Best to keep it encapsulated in the helper which returns layer/level
offset (and actually use that helper everywhere) rather than spreading
the logic around the code.

Also add a helper to find UBWC offset, to complete the encapsulation.

Signed-off-by: Rob Clark <robdclark@chromium.org>