minigbm: cros_gralloc: back out of scanout flag with more clarity

We interpret the following flags as GRALLOC_USAGE_HW_COMPOSER
as BO_USE_SCANOUT | BO_USE_RENDERING:

If the combination is not supported by the driver, let's check if
the gralloc flag is present before we back out for clarity.

BUG=chromium:616275
TEST=compiles

Change-Id: If546ce653f1088cbb1d31cf2f90aba0621ed8b4a
Reviewed-on: https://chromium-review.googlesource.com/451978
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/cros_gralloc/cros_alloc_device.cc b/cros_gralloc/cros_alloc_device.cc
index 97c8186..7f0a50a 100644
--- a/cros_gralloc/cros_alloc_device.cc
+++ b/cros_gralloc/cros_alloc_device.cc
@@ -22,7 +22,7 @@
 	supported = drv_is_combination_supported(drv, drv_format, drv_usage,
 						 DRM_FORMAT_MOD_NONE);
 
-	if (!supported && (drv_usage & BO_USE_SCANOUT)) {
+	if (!supported && (usage & GRALLOC_USAGE_HW_COMPOSER)) {
 		drv_usage &= ~BO_USE_SCANOUT;
 		supported = drv_is_combination_supported(drv, drv_format,
 							 drv_usage,