drm/i915/pci: rename functions to have i915_pci prefix

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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210825150623.28980-1-jani.nikula@intel.com