drm/i915: Use I915_READ_FW for plane updates

Optimize the plane register accesses a little bit by grabbing
the uncore lock manually across the entire pile of accesses and
using I915_READ_FW().

This helps keep the pipe update vblank evade critical section
below our 100 usec deadline, particularly with lockdep enabled.
And in general we want to keep that critical section as short
as possible as it's executed with interrupts disabled.

Not all plane updates currently happen from within the vblank evade
critical section, so we must use the irqsave/irqrestore variants
of the spinlock functions in the plane hooks.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309154434.29303-5-ville.syrjala@linux.intel.comReviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>