drm/i915: fix the transition minimums for gen9+ watermarks

The transition minimum is 14 blocks for gens 9 and 10, and 4 blocks
for gen 11. This minimum value is supposed to be added to the
configurable trans_amount. This matches both BSpec and additional
information provided by our HW engineers.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-3-paulo.r.zanoni@intel.com