dri_util: don't fail when not supporting ARB_compatibility with GL3.1

Currently, any driver that does not support the ARB_compatibility
extension will fail on GL3.1 context creation if the application does
not request the forward-compatiblity flag.

Restore the original check which changes mesa_api to API_OPENGL_CORE,
only when:
 - GL3.1 is requested, without the forward-compatiblity flag.
 - driver does not support ARB_compatibility - as deduced by
max_gl_compat_version.

Fixes: a0c8b49284 ("mesa: enable OpenGL 3.1 with ARB_compatibility")

v2:
 - Improve commit log (Emil).
 - Provide a correct explanation on the features documentation (Ian).

Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

docs/features.txt

@@ -24,10 +24,13 @@ not started
 
 # OpenGL Core and Compatibility context support
 
-Some drivers do not support the Compatibility profile or ARB_compatibility.
-Such drivers are limited to OpenGL 3.0 if the Core profile is not requested
-by applications. Some of the later GL features are exposed in the 3.0 context
-as extensions.
+Some drivers do not support the Compatibility profile or the
+ARB_compatibility extensions.  If an application does not request a
+specific version without the forward-compatiblity flag, such drivers
+will be limited to OpenGL 3.0.  If an application requests OpenGL 3.1,
+it will get a context that may or may not have the ARB_compatibility
+extension enabled.  Some of the later GL features are exposed in the 3.0
+context as extensions.
 
 
 Feature                                                 Status