drm/i915: Drop extern qualifiers from header function prototypes

Follow dim checkpatch recommendation so it doesn't complain on that now
and again on header file modifications.

v2: drop testing leftover (Chris)

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.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/20190712112429.740-2-janusz.krzysztofik@linux.intel.com