drm/amd/display: Possible divide by zero in set_speed()

If "speed" is zero then we use it as a divisor to find "prescale".  It's
better to move the check for zero to the very start of the function.

Fixes: 9eeec26a ("drm/amd/display: Refine i2c frequency calculating sequence")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>