mesa: Remove ARB_draw_buffers extension enable flag

mesa: Remove ARB_draw_buffers extension enable flag

All drivers in Mesa have supported this extension for eons.  This
extension is an optional features in desktop OpenGL (via
GL_ARB_draw_buffers) and OpenGL ES 2.x (via GL_NV_draw_buffers).

The extension is not usable in OpenGL ES 1.x.  There is no
glDrawBuffers* entry point in OpenGL ES 1.x contexts, and glGet*v
generate errors when MAX_DRAW_BUFFERS or DRAW_BUFFERi is queried.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>