[Why]
In some cases, prefetch mode 0 is unsupported but prefetch 1 is
supported. Due to previous change always forcing prefetch mode to 0, we
are failing bandwidth validation in cases where we should not.

[How]
By default try prefetch mode 0 but in the case validation fails, attempt
to do prefetch mode 1 to see if it is supported.
Signed-off-by: Isabel Zhang <isabel.zhang@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>