drm/i915/irq: Refactor gen11 display interrupt handling

drm/i915/irq: Refactor gen11 display interrupt handling

Let's move handling and reset for gen11 display IRQs to their own
functions, similar to how we deal with GT interrupts.  This will make
the top-level functions a bit easier to read and potentially make things
easier to deal with in the future if new platforms wind up needing
different display handling logic.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202171608.3361125-1-matthew.d.roper@intel.com
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>