Attempt to fix the issue reported by Alan Grimes with DRM_RADEON_TEXTURE

Attempt to fix the issue reported by Alan Grimes with DRM_RADEON_TEXTURE
erroring out when it shouldn't.  The errno could be changed by usleep() between
the ioctl call and the loop check, if a signal was received.  This could cause
an EAGAIN return from the DRM_RADEON_TEXTURE ioctl to not loop again.
Instead of checking errno, check thevalue of ret itself, since it is a saved
(and sign-flipped) copy of errno from the ioctl call.