drivers: ssd16xx: Implement blanking support

Implement support for display blanking by inhibiting display refresh
while blanking is on. Writes to the display are still sent to the
internal RAM in the display controller. Once blanking is turned off,
the contents of the RAM are written to the display. This behavior is
makes it possible to compose a frame buffer in the display controller
without flickering.

Other e-paper drivers such as the GD7965 already implement this
behavior.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>