Simplify Framebuffer::Session interface

This patch removes the 'Framebuffer::Session::release()' function from
the interface to make the mode-change protocol consistent with the way
the Rom_session interface handles ROM-module changes. That is, the
client acknowledges the release of its current dataspace by requesting a
new dataspace via the 'Framebuffer::Session::dataspace()' function.

Fixes #1057