drm: drop extern from function decls

It's the default storage class for functions, entirely redundant. And
a lot of these headers are a bit inconsistent due to organically
grown.

Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322083617.13361-2-daniel.vetter@ffwll.ch