s3c-fb: add support for display panning

Supports all bpp modes.

The PRTCON register is used to disable in-hardware updates of registers
that store start and end addresses of framebuffer memory.  This prevents
display corruption in case we do not make it before VSYNC with updating
them atomically.  With this feature there is no need to wait for a VSYNC
interrupt before each such update.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: InKi Dae <inki.dae@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>