OMAP: DSS2: Use dss_features framework on DSS2 code

Calls init functions of dss_features during dss_probe, and the following
features are made omapxxxx independent:
  - number of managers, overlays
  - supported color modes for each overlay
  - supported displays for each manager
  - global aplha, and restriction of global alpha for video1 pipeline
  - The register field ranges : FIRHINC, FIRVINC, FIFOHIGHTHRESHOLD
    FIFOLOWTHRESHOLD and FIFOSIZE

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>