drm/i915/bxt: Enable GMBUS IRQ

GMBUS interrupt has been moved to CPU side in BXT.
What this patch does is:
1. Enable GMBUS IRQ in de_post_install function
2. Handle this interrupt as a port interrupt in display irq
   handler

v2: Rebase on top of the for_each_pipe() change adding dev_priv as
    first argument (Damien).
v3: read BXT_DE_PORT_GMBUS IIR flag only on BXT on other platforms
    it's reserved (imre)
v4: (jani)
- remove redundant 'BXT GMBUS' comment
- fix formatting of BXT_DE_PORT_GMBUS definition

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> (v1)
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>