drm/i915: Make irq_received bool

irq_received is used as a boolean in i965_irq_handler(), so make it
bool. This also makes i965_irq_handler() closer to i915_irq_handler().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewd-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>