drm/i915: Use GEN8_MASTER_IRQ_CONTROL consistently

Use GEN8_MASTER_IRQ_CONTROL instead of DE_MASTER_IRQ_CONTROL or
MASTER_INTERRUPT_ENABLE with the GEN8_MASTER_IRQ register. They're
all bit 31 so there's no actual bug here, but let's be consistent
which name we use for the bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-2-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>