drm/i915: Use GEN5_IRQ_INIT() in vlv_display_irq_postinstall()

Replace the hand rolled IMR/IER setup in vlv_display_irq_postinstall()
with GEN5_IRQ_INIT(). Also rename the iir_mask to enable_mask to avoid
consusion since we no longer deal with IIR here.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460382992-28728-7-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>