drm/i915: Hold the pps mutex across the whole panel power enable sequence

Just grab the pps_mutex once and do all the pps panel startup operations
while holding the mutex instead of grabbing the mutex separately for
each individual step.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>