drm/i915/mchbar: WARN when accessing non-MCHBAR registers via intel_mchbar_read*()

drm/i915/mchbar: WARN when accessing non-MCHBAR registers via intel_mchbar_read*()

The intel_mchbar_read*() functions should only be used for
accessing MCHBAR registers. Warn if someone tries to use
them for other registers.

I suppose we could even have a dedicated type for MCHBAR
registers. But that is true for many other special register
types as well, and so far we haven't bothered adding any
special types apart from i915_mcr_reg_t.

v2: Print the register offset (Jani)
    Mention i915_mcr_reg_t (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260331154259.24600-5-ville.syrjala@linux.intel.com