configure: cleanup classic dri drivers handling

* Make sure that only drivers that are handled by configure.ac
are included in DRI_DIRS.
* Change with_dri_drivers default value to auto, and set enable
autodetection, when enable_opengl is on.

v2: Move "test" to the correct location.
v3: Squash DRI_DIRS handling before the switch statement.
Suggested by Ilia Mirkin

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
1 file changed