drm/i915: Extract intel_plane_{pin,unpin}_fb()

drm/i915: Extract intel_plane_{pin,unpin}_fb()

We've replicated the fb pin/unpin code in a few places. Pull it into
convenint helpers.

Slight change in locking behaviour as intel_cleanup_plane_fb() now
grab struct_mutex unconditionally.

v2: Change the locking to be symmetric between pin and unpin

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-6-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>