drm/i915: Fix latency==0 handling for level 0 watermark on skl+

drm/i915: Fix latency==0 handling for level 0 watermark on skl+

If the level 0 latency is 0 we can't do anything. Return an error
rather than success.

While this can't happen due to WaWmMemoryReadLatency, it can
happen if the user clears out the level 0 latency via debugfs.

v2: Clarify how how we can end here with zero level 0 latency (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-6-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>