OMAP3630: DSS2: Enable Pre-Multiplied Alpha Support

Enable dss to process color formats with pre-mulitplied alpha.
With this we can have alpha values defined for each pixel
and hence can have different blending values for each pixel.
sysfs entry has been created for this and pre-multiplied alpha
support is turned off by default.

Signed-off-by: Sudeep Basavaraj <sudeep.basavaraj@ti.com>
Signed-off-by: Rajkumar N <rajkumar.nagarajan@ti.com>
Signed-off-by: Samreen <samreen@ti.com>
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>