drm/i915: change to_mock() to an inline function

Since this function is defined in a header file, it should be
'static inline' instead of 'static'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191108051356.29980-1-yamada.masahiro@socionext.com