drm/i915/mchbar: Provide intel_mchbar_read*() abstraction

drm/i915/mchbar: Provide intel_mchbar_read*() abstraction

MCHBAR registers are a bit special in that:
- we access them through the mirror
- the mirror is read only on HSW+
- the mirror requires the actual MCHBAR to be enabled in device 0:0.0
- the mirror is gone on MTL+

So I'd prefer to treat MCHBAR registers as a bit special in
the code as well, and do all accesses to them via dedicated
functions. Prodive such functions in the form of
intel_mchbar_read*().

v2: Put the function arguments on one line
    No intel_uncore_read64() on xe, use intel_uncore_read64_2x32()
    Name the new function intel_mchbar_read64_2x32() as well

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260331154259.24600-3-ville.syrjala@linux.intel.com