drm/i915: Reset the interrupt mask on disabling interrupts

As we disable the interrupt during suspend, also reset the irq_mask to
short-circuit subsystems that later try to turn off their interrupt
source.

<4>[  101.816730] i915 0000:00:02.0: drm_WARN_ON(!intel_irqs_enabled(dev_priv))
<4>[  101.816853] WARNING: CPU: 3 PID: 4241 at drivers/gpu/drm/i915/i915_irq.c:343 ilk_update_display_irq+0xb3/0x130 [i915]

v2: Reset irq_mask for i8xx_irq_reset as well, and split patch to focus
on only i915->irq_mask

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201022114246.28566-1-chris@chris-wilson.co.uk