drm/i915: fix size_t greater or equal to zero comparison

Currently the check that the unsigned size_t variable i is >= 0
is always true because the unsigned variable will never be negative,
causing the loop to run forever.  Fix this by changing the
pre-decrement check to a zero check on i followed by a decrement of i.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: bfed6708 ("drm/i915: use vmap in shmem_pin_map")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201002170354.94627-1-colin.king@canonical.com

drivers/gpu/drm/i915/gt/shmem_utils.c

@@ -73,7 +73,7 @@ void *shmem_pin_map(struct file *file)
 	mapping_set_unevictable(file->f_mapping);
 	return vaddr;
 err_page:
-	while (--i >= 0)
+	while (i--)
 		put_page(pages[i]);
 	kvfree(pages);
 	return NULL;