drm/amdkfd: add new flag for uncached GPU mapping

The macro is for memory mapped by GPU as uncached.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Oak Zeng <Oak.Zeng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>