drm/vc4: Remove unused mode variable

"89d13761

 drm/vc4: Add support for margins to fkms" removed
the requirement for having the mode structure from vc4_plane_to_mb,
but didn't remove it as a local to the function, causing a
compiler warning.

Remove the unused variable.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>