drm/i915/driver: add i915_driver_ prefix to functions

Add the i915_driver_ prefix to the switcheroo functions in
i915_driver.[ch].

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211111101304.13094-3-jani.nikula@intel.com