drm/i915: Rename is_16gb_dimm to wm_lv_0_adjust_needed

As it now it is always required for GEN12+ the is_16gb_dimm name
do not make sense for GEN12+.

v2:
- Updated comment on top of "dram_info->wm_lv_0_adjust_needed =
!IS_GEN9_LP(i915);"

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128164312.91160-3-jose.souza@intel.com