drm/i915: Update PM interrupts before updating the freq

drm/i915: Update PM interrupts before updating the freq

Currently we update the freq before masking the interrupts, which can
allow new interrupts to occur before the frequency has changed. These
extra interrupts might waste some cpu cycles. This patch corrects
this by masking interrupts prior to updating the frequency.

Note from Chris:
"Well it won't waste CPU cycles as the interrupt is also masked by the
threshold limits, but there should be no harm at all in reordering the
patch so, and it does make a certain amount of sense."

Signed-off-by: Deepak S <deepak.s@intel.com>
Signed-off-by: Praveen Paneri <praveen.paneri@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add note from Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>