drm/i915/bxt: Add BXT support in gen8_irq functions

This patch adds conditional checks in gen8_irq functions
to support BXT. Most of the checks just look for PCH split
availability, and block the call to PCH interrupt functions if
not available.

v2: (jani)
- drop redundant TODO comment about PCH IRQ flags on BXT
- check HAS_PCH_SPLIT instead of IS_BROXTON when handling PCH specific
  IRQ events in gen8_irq_handler()
- check HAS_PCH_SPLIT before calling the function instead of a
  corresponding early return within the called function for
  ibx_irq_reset(), ibx_irq_pre_postinstall(), ibx_irq_postinstall()
v3: (jani)
- in ironlake_irq_postinstall() and ironlake_irq_reset() HAS_PCH_SPLIT
  is always true, so drop the check for it

Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Shashank Sharma <ppashank.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>