The system to allow panel drivers to exists as attached to ctrl drivers
did never work very well. It is not useed, and this patch removes it to
make the driver cleaner.

For now, controller drivers need to include also the panel driver code. In
the future a proper mechanism for this should be developed, perhaps by
creating busses for controllers.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

Rename and export dsi_vc_enable_hs() so that the display drivers can
control the mode of the DSI link.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

Add helper functions for most common DCS read and write operations.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

The core files of DSS2. DSS2 commits are split a bit artificially to
make the individual commits smaller, and DSS2 doesn't compile properly
without the rest of the core commits. This shouldn't be a problem, as no
configuration uses DSS2 yet.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>