mesa: Generate correct error code in glDrawBuffers()

mesa: Generate correct error code in glDrawBuffers()

OpenGL 3.3 spec expects GL_INVALID_OPERATION:
 "For both the default framebuffer and framebuffer objects, the
  constants FRONT, BACK, LEFT, RIGHT, and FRONT AND BACK are not
  valid in the bufs array passed to DrawBuffers, and will result
  in the error INVALID OPERATION."

But OpenGL 4.0 spec changed the error code to GL_INVALID_ENUM:
 "For both the default framebuffer and framebuffer objects, the
  constants FRONT, BACK, LEFT, RIGHT, and FRONT_AND_BACK are not
  valid in the bufs array passed to DrawBuffers, and will result
  in the error INVALID_ENUM."

This patch changes the behaviour to match OpenGL 4.0 spec
Fixes Khronos OpenGL CTS draw_buffers_api.test.

V2: Update the comment in code.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>