mesa: Add a _mesa_problem to document a piglit failure on i965.

mesa: Add a _mesa_problem to document a piglit failure on i965.

Having figured out what was going on with piglit fbo-depth copypixels
GL_DEPTH_COMPONENT32F (falling all the way back to swrast on CopyPixels to
a float depth buffer), I'm not inclined to fix the problem currently but
it seems worth saving someone else the debug time.

Reviewed-by: Matt Turner <mattst88@gmail.com>