drm: move i915_drm.h under include/drm/intel

drm: move i915_drm.h under include/drm/intel

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

v2: Also fix comment in intel_pci_config.h (Ilpo)

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>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@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/0e344a72e9be596ac2b8b55a26fd674a96f03cdc.1717075103.git.jani.nikula@intel.com