drm/vc4: crtc: Move the txp_armed function to the TXP

The TXP driver is the only place where we need to set the txp_armed flag,
so let's move the function in the TXP driver.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/12b383e7b8462e281b00c0a21b2b50f13691bead.1591882579.git-series.maxime@cerno.tech