drm/i915: Hold RPM reference while setting freq limits through sysfs

This changes ensures device is active when frequency limits are changed.
This is needed as we are writing to register RPNSWREQ in intel_set_rps.
If not done, might lead to undesired errors like:
[ 1965.189137] [drm:fw_domains_get] *ERROR* blitter: timed out waiting for forcewake ack to clear.

v2: Added elaborate commit message. (Jani)
    Fixing RPM reference drop in early exit paths. (Ville)

Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454951831-11778-1-git-send-email-sagar.a.kamble@intel.com


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>