drm/i915: Move engine-related mmio init to engines_init_mmio

All the info we read in intel_device_info_init_mmio are engine-related
and since we already have an engine_init_mmio function we can just
perform the operations from there.

v2: clarify comment about forcewake requirements and pruning (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> #v1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708003952.21831-4-daniele.ceraolospurio@intel.com