drm/i915/execlists: Trim irq handler

drm/i915/execlists: Trim irq handler

I noticed that gcc was spilling the CSB to the stack, so rearrange the
code to be more compact. Spilling in this function is slightly more
interesting due to the mmio reads acting as memory barriers and so
end up flushing the stack spills. Still miniscule to having to do at
least the pair of uncached reads :(

function                                     old     new   delta
intel_lrc_irq_handler                       1039     878    -161

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170325201053.21306-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>