drm/i915/mtl: Eliminate subplatforms

Now that we properly match the Xe_LPG IP versions associated with
various workarounds, there's no longer any need to define separate MTL
subplatform in the driver.  Nothing in the code is conditional on MTL-M
or MTL-P base platforms.  Furthermore, I'm not sure the "M" and "P"
designations are even an accurate representation of which specific
platforms would have which IP versions; those were mostly just
placeholders from a long time ago.  The reality is that the IP version
present on a platform gets read from a fuse register at driver init; we
shouldn't be trying to guess which IP is present based on PCI ID
anymore.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Nemesa Garg <nemesa.garg@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230821180619.650007-18-matthew.d.roper@intel.com