drm/amd/display: make dm_dp_aux_transfer return payload bytes instead of size

The drm layer expects aux->transfer() to return the payload bytes read.
Currently dm_dp_aux_transfer() returns the payload size which does not
gets updated during the read, hence not giving the right data for the
drm layer to pars edid. This leads to the drm layer to conclude as the
edid is BAD and hence some monitors/devices dont get detected properly.

This patch changes the return type of dm_dp_aux_transfer() to actual
bytes read during DP_AUX_NATIVE_READ & DP_AUX_I2C_READ.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>