drm/radeon/dp: handle zero sized i2c over aux transactions (v2)

Needed for proper i2c over aux handling for certain
monitors and configurations (e.g., dp bridges or
adapters).

v2: add comments clarifying tx_size setting.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>