drm/i915/guc: Sanitory checks for platform that dont have GuC

drm/i915/guc: Sanitory checks for platform that dont have GuC

i915.enable_guc_loading/submission=2 forces the usage of GuC.
For platforms that do not have a GuC, asking the kernel to use a GuC
should not result in an error state. Do extra checks to see if the
platform even has a GuC or not, regardless of the kernel parameter.

v2: Based on Rodrigo's patch and Paulo's suggestion(Paulo, Rodrigo)
v3: Correct the Indentation(Jani, Paulo)
v4: Added the blank line(Jani, Paulo)
v5 (from Paulo): Remove the extra blank line.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97573
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Zanoni Paulo <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476488825-5673-1-git-send-email-anusha.srivatsa@intel.com