Atm we call the handlers for pending pipestat interrupt events even if
they aren't explicitly enabled by i915_enable_pipestat(). This isn't an
issue for events other than the vblank start event, since those are
always enabled anyways. Otoh, we enable the vblank start event
on-demand, so we'll end up calling the vblank handler at times when they
are disabled.

I haven't checked if this causes any real problem, but for consistency
and to remove some overhead we should still fix this by clearing /
handling only the enabled interrupt events. Also this is a dependency
for the upcoming VLV power domain patchset where we need to disable all
the pipestat interrupts whenever the display power well is off.

v2:
- inline the status->enable mask mapping (Ville)
- don't check for invalid PSR bit on platforms other than VLV (Ville)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Frob conflict due to different merge order.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>