drm/vc4: crtc: Skip the TXP

The vc4_set_crtc_possible_masks is meant to run over all the encoders
and then set their possible_crtcs mask to their associated pixelvalve.

However, since the commit 39fcb280 ("drm/vc4: txp: Turn the TXP into
a CRTC of its own"), the TXP has been turned to a CRTC and encoder of
its own, and while it does indeed register an encoder, it no longer has
an associated pixelvalve. The code will thus run over the TXP encoder
and set a bogus possible_crtcs mask, overriding the one set in the TXP
bind function.

In order to fix this, let's skip any virtual encoder.

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/20210507150515.257424-3-maxime@cerno.tech