drm/i915/gvt: Add header to use display offset functions in macros

Introduce gvt/display_helpers.h to make DISPLAY_MMIO_BASE and
INTEL_DISPLAY_DEVICE_*_OFFSET macros call exported display functions.
This lets GVT keep using existing register macros (e.g.,
TRANSCONF(display, pipe)) while ensuring offset calculations happen
through functions instead of accessing display internals.

Ideally, we would remove the display headers that define these macros,
but some macros in GVT still depend on them and have not yet been
ported. Keeping those headers leads to build conflicts, so as a
stopgap, we use temporary ifdef/undef blocks to override the macros
with API-backed versions. These will be removed once all dependent
macros are ported and the conflicting headers can be safely dropped.

Note:
TRANSCONF() expects a pipe index but some GVT callers pass a transcoder,
causing -Werror=enum-conversion.
Fix: cast to enum pipe in the GVT-side macro override.
This works for all cases as TRANSCODER_{A,B,C,D} all have 1:1 mapping to
PIPE_{A,B,C,D} except for TRANSCODER_EDP which is used in one place.
In any case, the cast preserves the previous behaviour.

v2:
 - Remove prefix `gvt/` while including the header file. (Jani)
 - Explain the rationale behind temporary ifdef/undefs and plan to drop
   them. (Jani).
v3:
 - Meld the patch to cast argument to enum pipe for the pipe-offset
   macro. (Jani)
 - Add a FIXME to highlight the cast. (Jani)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20251219060302.2365123-4-ankit.k.nautiyal@intel.com

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