drm/amd/display: Adjust cursor position

[why & how]
When the commit 9d84c7ef ("drm/amd/display: Correct cursor position
on horizontal mirror") was introduced, it used the wrong calculation for
the position copy for X. This commit uses the correct calculation for that
based on the original patch.

Fixes: 9d84c7ef ("drm/amd/display: Correct cursor position on horizontal mirror")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 8f9b23ab)
Cc: stable@vger.kernel.org