display: add `frame_incomplete` to `display_buffer_descriptor`

Introduces support for double-buffered/latched displays.
Currently, every write has to be presented to the user immediately,
which negates the advantage of latched displays to prevent frame tearing.

Now, GUI managers can indicate whether the current `display_write` call
is the last call of the frame or not, allowing displays to group writes
to a single present.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>