mesa: Fix bogus strncmp

mesa: Fix bogus strncmp

The string "[0]\0" is the same as "[0]" as far as the C string datatype
is concerned.  That string has length 3.  strncmp(s, length_3_string, 4)
is the same as strcmp(s, length_3_string), so make it be strcmp.

v2: Not the same as strncmp(..., 3).  Noticed by Ilia.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>