drm/i915: Move MCR_REG define to i915_reg_defs.h

Define MCR_REG() in the same header where i915_mcr_reg_t is defined,
like i915_reg_t and _MMIO(). It's a more natural place for such a
definition so it's not mixed with the registers for the platforms.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230224211221.1557268-1-lucas.demarchi@intel.com