drm/i915: drop i915_ prefix from enable_rc6, enable_fbc, enable_ppgtt parameters

Having to use i915.i915_foo is inconsistent and a bit on the verbose
side. Drop the prefix per Daniel's request, who also says this is not
ABI we need to maintain.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>