drm/i915: Initialize pm_intr_keep during intel_irq_init for GuC

Driver needs to ensure that it doesn't mask the PM interrupts, which are
unmasked/needed by GuC firmware. For that, Driver maintains a bitmask of
interrupts to be kept unmasked, pm_intr_keep.

pm_intr_keep was determined across GuC load. GuC gets loaded in different
scenarios and it is not going to change the pm_intr_keep so this patch
moves its setup to intel_irq_init.

This patch fixes incorrect RPS masking leading to UP interrupts triggered
even when at cur_freq=max and inversly for Down interrupts.

Cc: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488862355-9768-1-git-send-email-sagar.a.kamble@intel.com