drm/i915: split gen11_irq_handler to make it shareable

Split gen11_irq_handler() to receive as parameter the function
pointers. This allows to share the interrupt handler even if the enable/disable
functions are different.

Make sure it's always inlined to avoid the extra indirect call on the
hot path. Checking with gcc 9 this produce the exact same code as of
now:

$ size drivers/gpu/drm/i915/i915_irq*.o
   text	   data	    bss	    dec	    hex	filename
  47511	    560	      0	  48071	   bbc7	drivers/gpu/drm/i915/i915_irq.o
  47511	    560	      0	  48071	   bbc7	drivers/gpu/drm/i915/i915_irq_new.o

$ gdb -batch -ex 'file drivers/gpu/drm/i915/i915_irq.o' -ex 'disassemble gen11_irq_handler' > /tmp/old.s
$ gdb -batch -ex 'file drivers/gpu/drm/i915/i915_irq_new.o' -ex 'disassemble gen11_irq_handler' > /tmp/new.s
$ git diff --no-index /tmp/{old,new}.s
$

So, no change in behavior, just a simple refactor.

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024195122.22877-4-lucas.demarchi@intel.com