drm/i915/gen10: implement gen 10 watermarks calculations

They're slightly different than the gen 9 calculations.

v2: Remove TODO comment. Code matches recent spec.
v3: Rebase on top of latest skl code using new fp16.16 and
    fixing a logic issue. Auto rebase bot has apparently
    made some bad decisions that changed the logic of the
    code. (Noticed by Manesh, updated by Rodrigo).

Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811233825.32083-1-rodrigo.vivi@intel.com