drm/i915/dram: Move 16Gb DIMM detection fully to the skl/icl codepaths

drm/i915/dram: Move 16Gb DIMM detection fully to the skl/icl codepaths

We are incorrectly applying the 16Gb DIMM w/a (adding 1 extra
usec to WM0 latency) on MTL+ even though the w/a is only needed
for SKL/ICL.

The current way of setting this is up is a bit of a disaster:
1. always set has_16gb_dimms=true for all platforms except BXT/GLK
2. has_16gb_dimms potentially gets overwritten with something else
 * BXT/GLK don't do anything since we never set has_16gb_dimms to
   begin with
 * skl_get_dram_info() overwrites has_16gb_dimms with the actual
   detection results for SKL/ICL/derivatives
 * gen12_get_dram_info() (correctly) resets has_16gb_dimms
   for TGL/ADL/derivatives
 * xelpdp_get_dram_info() doesn't do anything, leaving
   has_16gb_dimms incorrectly set for MTL+

Clean up the whole mess by only setting has_16gb_dimms in the
SKL/ICL codepaths where we have the actual detection code for
it. This avois applying the w/a incorrectly on MTL+.

v2: Rewrite commit msg (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250902133113.18778-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>