[media] dm1105: remove unneeded not-null test

i2c_adap is a field of a struct and will always be allocated so
its address will never be null.
Suggested by coccinelle, manually verified.

Signed-off-by: Alexandru Juncu <alexj@rosedu.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>