drm/i915: rip out ring->irq_mask

We only ever enable/disable one interrupt (namely user_interrupts and
pipe_notify), so we don't need to track the interrupt masking state.

Also rename irq_enable to irq_enable_mask, now that it won't collide -
beforehand both a irq_mask and irq_enable_mask would have looked a bit
strange.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>