drm/i915: Stop inlining the execlists IRQ handler

As the handler is now quite complex, involving a few atomics, the cost
of the function preamble is negligible in comparison and so we should
leave the function out-of-line for better I$.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-11-chris@chris-wilson.co.uk