drm/i915: transition WMs ask for Selected Result Blocks

The transition watermarks ask for Selected Result Blocks (the real
value), not Result Blocks (the integer value). Given how ceilings are
applied in both the non-transition and the transition watermarks
calculations, we can get away with assuming that Selected Result
Blocks is actually Result Blocks minus 1 without any rounding errors.

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/20181004231600.14101-5-paulo.r.zanoni@intel.com