drm/i915/gmch: avoid unused variable warning

When CONFIG_PNP is disabled, the mchbar_addr variable is only written but
not read:

drivers/gpu/drm/i915/soc/intel_gmch.c: In function 'intel_alloc_mchbar_resource':
drivers/gpu/drm/i915/soc/intel_gmch.c:41:13: error: variable 'mchbar_addr' set but not used [-Werror=unused-but-set-variable]
   41 |         u64 mchbar_addr;
      |             ^~~~~~~~~~~

No idea why this showed up now, but it's easy to fix by changing the #ifdef to
an IS_ENABLED() check that the compiler can see through.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230612124408.521325-1-arnd@kernel.org