drm/i915: Remove user controllable DRM_ERROR for i915_getparam()

The GETPARAM ioctl writes to a user supplied address. If that address is
invalid, it is the user's error and not the driver's, so quietly report
EFAULT and don't blame ourselves with a DRM_ERROR.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-11-git-send-email-chris@chris-wilson.co.uk