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
The platform driver is used to access the features provided by the
Videocore mboxes, i.e., power configuration and framebuffer setup. The
framebuffer driver uses the platform interface to setup a screen mode of
1024x768.