gallium/dri2: Enable {GLX_ARB,EGL_KHR}_context_flush_control

This was disabled ages ago because it provoked bugs between us and
xserver about context creation attributes, hopefully those servers are
out of circulation by now, let's find out.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17756>

docs/features.txt

@@ -347,7 +347,7 @@ Khronos, ARB, and OES extensions that are not part of any OpenGL or OpenGL ES ve
   GL_OES_texture_half_float_linear                      DONE (freedreno, i965, r300, r600, radeonsi, nv30, nv50, nvc0, softpipe, llvmpipe, panfrost, v3d, zink, lima)
   GL_OES_texture_view                                   DONE (freedreno, i965/gen8+, r600, radeonsi, nv50, nvc0, softpipe, llvmpipe, v3d, zink)
   GL_OES_viewport_array                                 DONE (i965, nvc0, r600, radeonsi, softpipe, zink)
-  GLX_ARB_context_flush_control                         not started
+  GLX_ARB_context_flush_control                         DONE (all drivers)
   GLX_ARB_robustness_application_isolation              not started
   GLX_ARB_robustness_share_group_isolation              not started
 

docs/relnotes/new_features.txt

@@ -12,3 +12,5 @@ VK_EXT_shader_module_identifier on RADV
 variablePointers on lavapipe
 VK_EXT_multisampled_render_to_single_sampled on lavapipe
 VK_EXT_shader_subgroup_vote/ballot on lavapipe
+EGL_KHR_context_flush_control for all drivers
+GLX_ARB_context_flush_control for all drivers

src/gallium/frontends/dri/dri2.c

@@ -2366,6 +2366,7 @@ static const __DRIextension *dri_screen_extensions_base[] = {
    &dri2InteropExtension.base,
    &driBlobExtension.base,
    &driMutableRenderBufferExtension.base,
+   &dri2FlushControlExtension.base,
 };
 
 /**