drm/i915/mtl: Skip doubling channel numbers for LPDDR4/LPDDDR5

MTL LPDDR5 reported 16b with 8 channels. Previous platforms
reported 32b with 4 channels and hence needed a multiplication
by a factor of 2. Skip increasing the channels for MTL.

v2: Use version check instead of platform check(MattR)

Bspec: 64631
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/20221117213015.584417-2-radhakrishna.sripada@intel.com