Drop the unnecessary directory prefix.  This also makes
intel_display_device.h easier to use from the Xe driver.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230607150946.1996087-1-matthew.d.roper@intel.com