drm/i915/mtl: Add fake PCH for Meteor Lake

Correct the implementation trying to detect MTL PCH with
the MTL fake PCH id.

On MTL, both the North Display (NDE) and South Display (SDE) functionality
reside on the same die (the SoC die in this case), unlike many past
platforms where the SDE was on a separate PCH die. The code is (badly)
structured today in a way that assumes the SDE is always on the PCH for
modern platforms, so on platforms where we don't actually need to identify
the PCH to figure out how the SDE behaves (i.e., all DG1/2 GPUs as well as
MTL and LNL),we've been assigning a "fake PCH" as a quickhack that allows
us to avoid restructuring a bunch of the code.we've been assigning a
"fake PCH" as a quick hack that allows us to avoid restructuring a bunch
of the code.

Removed unused macros of LNL amd MTL as well.

v2: Reorder PCH_MTL conditional check (Matt Roper)
    Reverting to PCH_MTL for PICA interrupt(Matt Roper)

Signed-off-by: Haridhar Kalvala <haridhar.kalvala@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231219185233.1469675-1-haridhar.kalvala@intel.com