drm/i915/dram: Interpret 0xF populated-channel count as 16

Wa_16030862157:
Interpret 0xF populated-channel count as 16
The register MEM_SS_INFO_GLOBAL [Number of populated channels] field
definition is updated with an encoding for 16 channels.

For 16-channel configuration, program 1111b. A programmed value of 1111b
must be interpreted as 16 channels for memory bandwidth calculations.

The MEM_SS_INFO_GLOBAL populated-channel field is only 4 bits and cannot
encode 16, so on Xe3p the BIOS programs the saturated field value (0xf)
to indicate the fully-populated 16-channel config (4 memory controllers
x 4 channels). Interpret it as 16 and let the bandwidth math handle the
larger channel count.

v2: Limit the WA only till NVL (Suraj)
    Logical separation of WA (Vinod)

WA: 16030862157, 16030875223
Bspec: 69131, 79482

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patch.msgid.link/20260715143243.4141208-2-uma.shankar@intel.com

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