drm/radeon: Always store CRTC relative radeon_crtc->cursor_x/y values

We were storing viewport relative coordinates for AVIVO/DCE display
engines. However, radeon_crtc_cursor_set2 and radeon_cursor_reset pass
radeon_crtc->cursor_x/y as the x/y parameters of
radeon_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>