drm/i915/gen9: fix the WaWmMemoryReadLatency implementation

Bspec says:
  "The mailbox response data may not account for memory read latency.
   If the mailbox response data for level 0 is 0us, add 2 microseconds
   to the result for each valid level."

This means we should only do the +2 in case wm[0] == 0, not always.

So split the sanitizing implementation from the WA implementation and
fix the WA implementation.

v2: Add Fixes tag (Maarten).

Fixes: 367294be7c25 ("drm/i915/gen9: Add 2us read latency to WM level")
Cc: [email protected]
Cc: Vandana Kannan <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Paulo Zanoni <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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