drm/amd/display: Update swizzle mode enums

[Why]
Swizzle mode enum for DC_SW_VAR_R_X was existing,
but not mapped correctly.

[How]
Update mapping and conversion for DC_SW_VAR_R_X.

Reviewed-by: XiangBing Foo <XiangBing.Foo@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Cc: stable@vger.kernel.org
Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>