drm/i915: program iCLKIP on Lynx Point

The iCLKIP clock is used to drive the VGA pixel clock on the PCH. In order
to do so, it must be programmed to properly do the clock ticks according
to the divisor, phase direction, phase increments and a special auxiliary
divisor for 20MHz clock.

v2: calculate divisor values directly instead of relying on a table.

v3: merged a fix from Ben to properly check for invalid divider values.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>