drm/i915/icl: Do not fix dbuf block size to 512

GEN9/10 had fixed DBuf block size of 512. Dbuf block size is not a
fixed number anymore in GEN11, it varies according to bits per pixel
and tiling. If 8bpp & Yf-tile surface, block size = 256 else block
size = 512

This patch addresses the same.

v2 (from Paulo):
  - Make it compile.
  - Fix a few coding style issues.
v3:
  - Rebase on top of upstream patches
v4 (from Paulo):
  - Bikeshed if statements (James).

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: James Ausmus <james.ausmus@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/20180130134918.32283-3-paulo.r.zanoni@intel.com