Follow the usual naming conventions. While at it, fix i915_pci.h SPDX
license comment format and add header include guards.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210825150623.28980-1-jani.nikula@intel.com