drm/i915: Fix up vlv/chv display irq setup

The vlv/chv display irq setup was a bit of mess after I ran out of steam
when working on it last. Fix it up so that we just have a _reset() and
_postinstall() hooks for the display irqs, and use those consistently.

v2: Clear out pipestat_irq_mask[] and PIPE_FIFO_UNDERRUN_STATUS in
    vlv_display_irq_reset() (Imre)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1460476574-1921-1-git-send-email-ville.syrjala@linux.intel.com