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

When current delay is already at max delay, Let's disable the PM UP
THRESHOLD INTRRUPTS, so that we will not get further interrupts until
current delay is less than max delay, Also request for the PM DOWN
THRESHOLD INTRRUPTS to indicate the decrease in clock freq. and
viceversa for PM DOWN THRESHOLD INTRRUPTS.

v2: Use bool variables (Daniel)

v3: Fix Interrupt masking bit (Deepak)

v4: Use existing symbolic constants in i915_reg.h (Daniel)

v5: Add pm interrupt mask after new_delay calculation (Ville)

Signed-off-by: Deepak S <deepak.s@intel.com>
[danvet: Pass new_delay by value as suggested by Ville. Also appease
checkpatch.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>