drm/msm/sde: store mdss irq status in case of irq storm

Store the last MDSS IRQ status value to help identify the source
of any erroneous IRQs that might cause an IRQ storm. Also, check
return codes on IRQ mapping or handling and report any unhandled
IRQs to the IRQ system.

Change-Id: I585c97c01736216556eb389bda79548c1fda2b64
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>