drm/i915: Prevent using uninitialized MMIO funcs

drm/i915: Prevent using uninitialized MMIO funcs

For upcoming patches which will have GEN specific MMIO functions, we'll
need to initialize the uncore data structure earlier than we do today.

If we do not do this, the following will be problematic:

intel_uncore_sanitize
	intel_disable_gt_powersave
		gen6_disable_rps
			I915_WRITE(GEN6_RC_CONTROL, 0); <--- MMIO
intel_uncore_init // initializes MMIO

By initializing the function pointers first, we should be safe.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>