drm/nouveau: Add some generic I2C gadget detection code.

Clean up and move the external TV encoder detection code to
nouveau_i2c.c, it's also going to be useful for external TMDS and DDC
detection.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>