drm/i915/lspcon: Enable AUX interrupts for resume time initialization

drm/i915/lspcon: Enable AUX interrupts for resume time initialization

For LSPCON initialization during system resume we need AUX
functionality, but we call the corresponding encoder reset hook with all
interrupts disabled. Without interrupts we'll do a poll-wait for AUX
transfer completions, which adds a significant delay if the transfers
timeout/need to be retried for some reason.

Fix this by enabling interrupts before calling the reset hooks. Note
that while this will enable AUX interrupts it will keep HPD interrupts
disabled, in a similar way to the init time output setup code.

This issue existed since LSPCON support was added.

v2:
- Rebased on drm-tip.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480448429-27739-1-git-send-email-imre.deak@intel.com