xf86drm: Bound strstr() to the allocated data

We are reading at most sizeof(data) bytes, but then data may not contain
a terminating '\0', at least in theory, so strstr() may overflow the
stack allocated array.

Make sure that data always contains at least one '\0'.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>