drm/amd/display: Calculate ODM width using odm slice rect, not recout

[Description]
There are scenarios where ODM4:1 is used but the
surface is entirely outside of the first and last
ODM slice. In this case the recout.width for the
first and last slice is 0 because there's no overlap
with the surface and that ODM slice, but this causes
the x_pos for the cursor in this scenario to be
calculated incorrectly. Instead we should use the
ODM slice width instead of the recout width.

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>