i915: remove timespec_to_jiffies_timeout

This function has been unused since commit 5ed0bdf2 ("drm: i915:
Use nsec based interfaces"). Let's remove the definition as well now
to help get rid of all uses of 'timespec'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618153855.2126048-1-arnd@arndb.de