mdss: calculating length based on offsets from dts

If the len values are not mentioned in the device tree, then
calculate the length based on the corresponding offsets
Plus split mdss_sspp into: sspp_vig, sspp_rgb and sspp_dma.
Because in 8092, the lengths are different. Therefore, they
should be separate lengths in the generic implementation.

Change-Id: I0f83ffae15d1ae3e92a01f7f23215c94b23065c7
Signed-off-by: Zohaib Alam <zalam@codeaurora.org>