drm/i915: remove "extern" keywords from intel_drv.h functions

Since I already reorganized the header file, Daniel requested me to
remove those keywords. It seems "checkpath.pl --strict" also doesn't
like "extern" on header files.

At least now we're consistent :)
Requested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>