drm/i915/display: Add APIs to be used by gvt to get the register offsets

drm/i915/display: Add APIs to be used by gvt to get the register offsets

GVT code uses macros for register offsets that require display internal
structures. This makes clean separation of display code and
modularization difficult.

Introduce APIs to abstract offset calculations:
- intel_display_device_pipe_offset()
- intel_display_device_trans_offset()
- intel_display_device_cursor_offset()
- intel_display_device_mmio_base()

These APIs return absolute base offsets for the respective register
groups, allowing GVT to compute MMIO addresses without using internal
macros or struct fields. This prepares the path to separate
display-dependent code from i915/gvt/*.

v2:
- Build GVT APIs only when GVT is actually enabled. (Jani)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> (#v1)
Link: https://patch.msgid.link/20251219060302.2365123-3-ankit.k.nautiyal@intel.com