drm/i915: rename init/fini/prune uncore functions

Add "_mmio" postfix to be consistent from the init/fini phase they're
called from.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190402201032.15841-2-daniele.ceraolospurio@intel.com