i965: Do not set front_buffer_dirty if there is no front buffer

i965: Do not set front_buffer_dirty if there is no front buffer

Otherwise there will be a warning:
 "libEGL warning: FIXME: egl/x11 doesn't support front buffer rendering."

Happens with EGL_KHR_surfaceless_context:

 eglMakeCurrent(egl_display, EGL_NO_SURFACE, EGL_NO_SURFACE, egl_context)
 eglMakeCurrent(egl_display, egl_surface, egl_surface, egl_context)
 glFlush() // Here will be a warning

Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1525
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3628>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3628>