drm/i915/overlay: Split 'struct intel_overlay'

drm/i915/overlay: Split 'struct intel_overlay'

Split the i915 driver specific bits from 'struct intel_overlay'
into a seaarate 'struct i915_overlay'. The latter will move to
the i915 side of the parent vs. display driver split.

The display side will also need to know the virtual address of
the register map. That now gets passed as the return value
from i915_overlay_setup(), so that the display side doesn't
need to know how the mapping was achieved.

v2: Use kzalloc_obj()

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-17-ville.syrjala@linux.intel.com