drm/i915: refer to pin instead of port in the intel_i2c.c interfaces

Rename intel_gmbus_is_port_valid to intel_gmbus_is_valid_pin, and rename
port parameters to pin as well. This matches usage all around, as
usually a pin is passed to the validity check function. No functional
changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>