drm/i915: Disable C3 when enabling vblank interrupts on i945gm

The AGPBUSY thing doesn't work on i945gm anymore. This means
the gmch is incapable of waking the CPU from C3 when an interrupt
is generated. The interrupts just get postponed indefinitely until
something wakes up the CPU. This is rather annoying for vblank
interrupts as we are unable to maintain a steady framerate
unless the machine is sufficiently loaded to stay out of C3.

To combat this let's use pm_qos to prevent C3 whenever vblank
interrupts are enabled. To maintain reasonable amount of powersaving
we will attempt to limit this to C3 only while leaving C1 and C2
enabled.

v2: Use READ_ONCE() (Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30364
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322180804.3300-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>