drm/amdgpu: Store CRTC relative amdgpu_crtc->cursor_x/y values

We were storing viewport relative coordinates. However, crtc_cursor_set2
and cursor_reset pass amdgpu_crtc->cursor_x/y as the x/y parameters of
cursor_move_locked, which would break if the CRTC isn't located at
(0, 0).

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>