drm/i915: Rewrite GMCH irq handlers to avoid loops

Eliminate the loops from the gen2-3 irq handlers. Since we don't use
MSI anymore on these platforms, and thus the CPU interrupt will be level
triggered, we shouldn't need to play any tricks with IER to induce edges
from IIR. IIR itself still detects only edges from PIPESTAT & co. on
gen4 but since IIR is double buffered and we only clear one bit per irq
handler invocation we can use the normal "clear PIPESTAT & co. -> clear
IIR" approach to ack the interrupts. On gen2 everything is level
triggered, and gen3 presumably follows either the gen2 or gen4 approach
since nothing else would really make sense.

v2: Drop the IER tricks since we no longer use MSI

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-12-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>