drm/i915/irq: un-inline functions to avoid i915_drv.h include

Avoid including the i915_drv.h mega header from other header files to
make further header cleanup easier.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/45431103f6cbd4322808907154336fdc176ff20d.1565085691.git.jani.nikula@intel.com