drm/radeon: fix endian issues with DP handling (v3)

The atom interpreter expects data in LE format, so
swap the message buffer as apprioriate.

v2: properly handle non-dw aligned byte counts.
v3: properly handle remainder

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Dong He <hedonghust@gmail.com>
Cc: stable@vger.kernel.org