drm/i915: Move vlv/chv display irq code to a more logical place

Reshuffle the code a bit to move the vlv/chv display irq functions away
from the main irq hooks, next to the other sub (de,gt,etc.) hooks.

v2: Rebased due to changes in vlv_display_irq_reset()

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/1460476604-2035-1-git-send-email-ville.syrjala@linux.intel.com