drm/i915/display: Add missing include to intel_vga.c

This compilation unit uses udelay() function without including
it's header file. Fix that to break dependency on other code.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240520181814.2392-2-michal.wajdeczko@intel.com