drm/i915/bxt: DDI Hotplug interrupt setup

In BXT, DDI hotplug control has been moved to CPU from PCH.
This patch adds a new IRQ setup function for BXT which:
1. Checks which HPD ports are requested to be enabled by encoders.
2. Enables those ports in the hot plug control register.
3. Un-masks these port interrupts in the IMR register.
4. Enables these port interrupts in the IER register.

V3: Kept the default HPD filter count to default (500 us) as per
    satheesh's comment
v4: Remove unused HPD filter defines (Damien)
v5: warn if trying to setup HPD on port A (imre)
v6: fix order of definitions for register bitfields (Daniel)
v7: (jani)
- define the size of the hpd_bxt array explicitly for bound checking
- use for_each_intel_encoder instead of open coding it
- fix format/order of definitions for BXT_HOTPLUG_CTL reg bitfields

Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com> (v4)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>