drm/vc4: txp: Don't set TXP_VSTART_AT_EOF

The TXP_VSTART_AT_EOF will generate a second VSTART signal to the HVS.
However, the HVS waits for VSTART to enable the FIFO and will thus start
filling the FIFO before the start of the frame.

This leads to corruption at the beginning of the first frame, and
content from the previous frame at the beginning of the next frames.

Since one VSTART is enough, let's get rid of it.

Fixes: 008095e0 ("drm/vc4: Add support for the transposer block")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>