drm/i915: Add REG_WRITE_FOOTER

Add a REG_WRITE_FOOTER macro as a counterpart to the REG_WRITE_HEADER.
The current code has the spin_lock() in the HEADER, but the
spin_unlock() is open coded, which looks rather confusing on the first
glance. A bit of additional symmetry might help.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>