drm/i915/mtl: Fix dram info readout

MEM_SS_INFO_GLOBAL Register info read from the hardware is cached in val. However
the variable is being modified when determining the DRAM type thereby clearing out
the channels and qgv info extracted later in the function xelpdp_get_dram_info. Preserve
the register value and use extracted fields in the switch statement.

Fixes: 825477e7 ("drm/i915/mtl: Obtain SAGV values from MMIO instead of GT pcode mailbox")
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221117213015.584417-1-radhakrishna.sripada@intel.com