drm/i915/display: move display funcs under modeset sub-struct

drm/i915/display: move display funcs under modeset sub-struct

Move generic crtc-ish modeset related functions under a new modeset
sub-struct of struct intel_display. Rename struct intel_display_funcs to
intel_modeset_funcs to make it a little bit more specific. Remove the
funcs sub-struct.

The funcs sub-struct of struct intel_display seems unnecessary. Instead
of display->funcs.FEATURE, prefer display->FEATURE.funcs.

Reviewed-by: Nemesa Garg <nemesa.garg@intel.com>
Link: https://patch.msgid.link/7849de3a5c9755639c179917c6a298de9ac832c0.1777537663.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>