client: Fully flush during blocking dispatch

wl_display_flush() may fail with EAGAIN which means that not all data
waiting in the buffer has been flushed. We later block until there is
data to read, which could mean that we block on input from the
compositor without having sent out all data from the client. Avoid this
by fully flushing the socket before starting to wait.

This commit also changes the array length of the struct pollfd array
from 2 to 1, as only one element was ever used.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>