s3c-fb: automatically calculate pixel clock when none is given

Add a simple algorithm which calculates the pixel clock based on the video
mode parameters.  This is only done when no pixel clock is supplied
through the platform data.

This allows drivers to omit the pixel clock data and thus share the
algorithm used for calculating it.

Signed-off-by: Maurus Cuelenaere <mcuelenaere@gmail.com>
Cc: Pawel Osciak <p.osciak@samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: InKi Dae <inki.dae@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Tested-by: Donghwa Lee <yiffie9819@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>