drm/amd/display: Add bit swap helper based on endianness

Christian Koenig pointed out a code duplication related to bit swap in
case of big-endian manipulation. This commit adds a helper for handling
this verification and reduces the requirement of replicate some part of
the code.

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Wyatt Wood <Wyatt.Wood@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>