drm/vc4: txp: Properly set the possible_crtcs mask

The current code does a binary OR on the possible_crtcs variable of the
TXP encoder, while we want to set it to that value instead.

Cc: <stable@vger.kernel.org> # v5.9+
Fixes: 39fcb280

 ("drm/vc4: txp: Turn the TXP into a CRTC of its own")
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507150515.257424-2-maxime@cerno.tech