Fix drmSetMaster error reporting

Unlike libdrm's other drm*() wrapped DRM ioctls, drmSetMaster() and
drmDropMaster() just return -1 on error instead of returning errno.

Thus, to get the relevent error string, we must pass errno to strerror(),
not ret.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>

BUG=chromium-os:38551
TEST=See steps in crosbug.com/38551
/var/log/xorg/Xorg.0.log should now have a message like:
 [  1691.477] (EE) ARMSOC(0): ERROR: Cannot get DRM master:  Invalid argument

Change-Id: Ib3f59be04a806a93607b11f3b057e5ba4977e247
Reviewed-on: https://gerrit.chromium.org/gerrit/42590
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
1 file changed