drm: fix drm_get_max_iomem type mismatch

When comparing two variables with min()/max(), they should be the same type:

drivers/gpu/drm/drm_memory.c: In function 'drm_get_max_iomem':
include/linux/kernel.h:821:16: error: comparison of distinct pointer types lacks a cast [-Werror]
  (void) (&max1 == &max2);

This makes the local variable in drm_get_max_iomem make the type
from resource->end.

Fixes: 82626363 ("drm: add func to get max iomem address v2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222114804.1394300-1-arnd@arndb.de

drivers/gpu/drm/drm_memory.c

@@ -153,7 +153,7 @@ EXPORT_SYMBOL(drm_legacy_ioremapfree);
 u64 drm_get_max_iomem(void)
 {
 	struct resource *tmp;
-	u64 max_iomem = 0;
+	resource_size_t max_iomem = 0;
 
 	for (tmp = iomem_resource.child; tmp; tmp = tmp->sibling) {
 		max_iomem = max(max_iomem,  tmp->end);