drm/i915: Fix I2C adapter registration

Fix many small bugs in I2C adapter registration:
* Properly reject unsupported GPIO pin.
* Fix improper use of I2C_NAME_SIZE (which is the size of
  i2c_client.name, not i2c_adapter.name.)
* Prefix adapter names with "i915" so that the user knows what the
  I2C channel is connected to.
* Fix swapped characters in the string used to name the GPIO-based
  adapter.
* Add missing comma in gmbus name table.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>