drm/i915/gvt: Use the appropriate header for the DPLL macro

drm/i915/gvt: Use the appropriate header for the DPLL macro

The macro `DPLL_ID_SKL_DPLL0` is defined in
display/intel_dpll_mgr.h. Previously, GVT included the header
display/intel_display_core.h` because other macros also depended on it.
After porting those macros to use the new APIs, the only remaining
dependency was for the DPLL macro.

Replace the indirect include with the correct header and drop
intel_display_core.h to reduce unnecessary dependencies.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20251219060302.2365123-6-ankit.k.nautiyal@intel.com