drm/i915: kerneldoc for interrupt enable/disable functions

Just start with the basics for now.

Since there's a lot of different functionality in i915_irq.c I've
decided to split it into different sections and pull in just the
relevant functions. Splitting into different files looks like a lot
more work since the interrupt handlers do an awful lot of reuse all
over.

v2: Rebase onto changed function names.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>