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

drivers/gpu/drm/i915/soc/intel_gmch.c

@@ -47,11 +47,9 @@ intel_alloc_mchbar_resource(struct drm_i915_private *i915)
 	mchbar_addr = ((u64)temp_hi << 32) | temp_lo;
 
 	/* If ACPI doesn't have it, assume we need to allocate it ourselves */
-#ifdef CONFIG_PNP
-	if (mchbar_addr &&
+	if (IS_ENABLED(CONFIG_PNP) && mchbar_addr &&
 	    pnp_range_reserved(mchbar_addr, mchbar_addr + MCHBAR_SIZE))
 		return 0;
-#endif
 
 	/* Get some space for it */
 	i915->gmch.mch_res.name = "i915 MCHBAR";