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

v2: Also change Documentation/gpu/i915.rst (Andi)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.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/a8c07233a8234858eb6711140482ef8db4c91cf4.1717075103.git.jani.nikula@intel.com