drm/amdgpu: implement mmio byte access helper for MB

mailbox registers can be accessed with a byte boundry according
to BIF team, so this patch prepares register byte access
and will be used by following patches.

Actually, for mailbox registers once the byte field is touched even not changed,
the mailbox behaves, so we need the byte width accessing to those sort of regs.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Pixel Ding <Pixel.Ding@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>