drm/i915: Add intel_gpu_freq() and intel_freq_opcode()

Rename the vlv_gpu_freq() and vlv_freq_opecode() functions to have
an intel_ prefix, and handle non-VLV/CHV platforms in them as well.
Leave the vlv_ names around for now since they're currently used.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>