gallium: Add PIPE_CAP_DEMOTE_TO_HELPER_INVOCATION

gallium: Add PIPE_CAP_DEMOTE_TO_HELPER_INVOCATION

To enable EXT_demote_to_helper_invocation:

    This extension adds a "demote" keyword that is similar to "discard" but
    only suppresses subsequent writes and outputs to the framebuffer, and
    does not terminate the execution of the invocation. For the remainder
    of the execution, the invocation is "demoted" to act like a helper
    invocation.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>