drm: move intel-gtt.h under include/drm/intel

Clean up the top level include/drm directory by grouping all the Intel
specific files under a common subdirectory.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ae224504d99cc6428da6dced9dcde2b7953624ef.1717075103.git.jani.nikula@intel.com