drm/i915/irq: split ILK display irq handling

drm/i915/irq: split ILK display irq handling

Split out display irq handling on ilk. Since the master IRQ enable is in
DEIIR, we'll need to do this in two parts. First, add
ilk_display_irq_master_disable() to disable master and south interrupts,
and second, add (repurposed) ilk_display_irq_handler() to finish display
irq handling.

It's not the prettiest thing you ever saw, but improves separation of
display irq handling. And removes HAS_PCH_NOP() and DISPLAY_VER() checks
from core irq code.

v2:
- Separate ilk_display_irq_master_enable() (Ville)
- Use _fw mmio accessors (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/e8ea7c985c3f3a80870f3333bde2e1bf30d653b0.1758637773.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>