drm/i915: kill uncore_to_i915

Let's get rid of it before it proliferates, since with split GT/Display
uncores the container_of won't work anymore.

I've kept the rpm pointer as well to minimize the pointer chasing in the
MMIO accessors.

v2: swap parameter order for intel_uncore_init_early (Tvrtko)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620010021.20637-4-daniele.ceraolospurio@intel.com