drm/i915/gsc: use system include style for drm headers

Use <> instead of "" for including headers from include/.

Fixes: 8a9bf295 ("drm/i915/gsc: add initial support for GSC proxy")
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230525094942.941123-1-jani.nikula@intel.com