[media] hva: fix sparse warnings

drivers/media/platform/sti/hva/hva-v4l2.c:43:22: warning: symbol 'hva_encoders' was not declared. Should it be static?
drivers/media/platform/sti/hva/hva-v4l2.c:1401:24: warning: symbol 'hva_driver' was not declared. Should it be static?

Make these static.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>