minigbm: Fix consistency in return values

minigbm functions don't use errno; however drmIoctl does. So we need
to return -errno instead of returning exactly what drmIoctl returns.

BUG=none
TEST=builds

Change-Id: I20e00141782ac1407133ee72259fe43381954d26
Reviewed-on: https://chromium-review.googlesource.com/1534878
Commit-Ready: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
diff --git a/vc4.c b/vc4.c
index 71e73ea..6edd967 100644
--- a/vc4.c
+++ b/vc4.c
@@ -6,6 +6,7 @@
 
 #ifdef DRV_VC4
 
+#include <errno.h>
 #include <stdio.h>
 #include <string.h>
 #include <sys/mman.h>
@@ -49,7 +50,7 @@
 	ret = drmIoctl(bo->drv->fd, DRM_IOCTL_VC4_CREATE_BO, &bo_create);
 	if (ret) {
 		drv_log("DRM_IOCTL_VC4_GEM_CREATE failed (size=%zu)\n", bo->total_size);
-		return ret;
+		return -errno;
 	}
 
 	for (plane = 0; plane < bo->num_planes; plane++)