drm/i915/icl: Fail flip if ddb allocated are less than min display buffer needed

ICL require DDB allocation of plane to be more than "minimum display
buffer needed" for each level in order to enable WM level.

This patch implements and consider the same while allocating DDB
and enabling WM.

Changes Since V1:
 - rebase
Changes Since V2:
 - Remove extra parentheses
 - Use FP16.16 only when absolutely necessary (Paulo)
Changes Since V3:
 - Rebase
Changes since v4 (from Paulo):
 - Coding style issue.
Changes since v5 (from Paulo):
 - Do the final checks according to BSpec.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130134918.32283-4-paulo.r.zanoni@intel.com