display/amd: decrease message verbosity about watermarks table failure

A number of BIOS versions have a problem with the watermarks table not
being configured properly.  This manifests as a very scary looking warning
during resume from s0i3.  This should be harmless in most cases and is well
understood, so decrease the assertion to a clearer warning about the problem.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>