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>
Cc: Bjorn Helgaas <bhelgaas@google.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/a19cebc0f03588b9627dcaaebe69a9fef28c27f0.1717075103.git.jani.nikula@intel.com