drm/i915: Disable display interrupts during display IRQ handler

During the Display Interrupt Service routine the Display Interrupt
Enable bit must be disabled, The interrupts handled, then the
Display Interrupt Enable bit must be set to prevent possible missed
interrupts.

Bspec: 49212
V2: Change Title to remove SDE reference.
V3: Fix TAB spacing.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Aditya Swarup <aditya.swarup@intel.com>

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191121201455.2558-1-clinton.a.taylor@intel.com