drm/i915: Implement 16GB dimm wa for latency level-0

Memory with 16GB dimms require an increase of 1us in level-0 latency.
This patch implements the same.
Bspec: 4381

changes since V1:
 - s/memdev_info/dram_info
 - make skl_is_16gb_dimm pure function
Changes since V2:
 - make is_16gb_dimm more generic
 - rebase
Changes since V3:
 - Simplify condition (Maarten)

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180831110942.9234-1-mahesh1.kumar@intel.com