Revert "drm/i915: Disable/Enable PM Intrrupts based on the current freq."

This reverts commit 27544369.

Conflicts:
	drivers/gpu/drm/i915/i915_irq.c

The partial application of interrupt masking without regard to other
pathways for adjusting the RPS frequency results in completely disabling
the PM interrupts. This leads to excessive power consumption as the GPU
is kept at max clocks (until the failsafe mechanism fires of explicitly
downclocking the GPU when all requests are idle). Or equally as bad for
the UX, the GPU is kept at minimum clocks and prevented from upclocking
in response to a requirement for more power.

Testcase: pm_rps/blocking
Cc: Deepak S <deepak.s@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>