drm/i915: fix sparse warnings: move 'extern' decls to header file

Move 'extern'-decls from "intel_dvo.c" to "dvo.h", as "dvo.h" is
included by and only by files where the symbols are either defined or
used.

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>