dsi_vc_send_null() currently sends a long packet with data type
MIPI_DSI_NULL_PACKET and packet length 4. Modify it to send a zero
length long packet. This leads to sending only the long packet header
and no payload packets and hence the transfer completes faster.

The function can be modified later if there is a need to send null
packets of a non-zero length.
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>