Fix the following build warnings.

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:
	In function ‘dm_update_mst_vcpi_slots_for_dsc’:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6242:46:
	warning: variable ‘old_con_state’ set but not used

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:
	In function ‘amdgpu_dm_commit_cursors’:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:7709:44:
	warning: variable ‘new_plane_state’ set but not used

The variables were introduced to be used in iterators, but not used.
Use other iterators which don't require the unused variables.

Fixes: 8ad27806 ("drm/amd/display: Disable cursors before disabling planes")
Fixes: 29b9ba74 ("drm/amd/display: Recalculate VCPI slots for new DSC connectors")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>