drm/i915: Add missing locking to primary plane handlers

intel_primary_plane_{setplane,disable} were lacking struct_mutex locking
around their GEM operations.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reported-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>