OMAP: DSS2: Add GLOBAL_ALPHA & PRE_MULT_ALPHA to ovl caps

Add OMAP_DSS_OVL_CAP_GLOBAL_ALPHA and OMAP_DSS_OVL_CAP_PRE_MULT_ALPHA to
overlay capabilities. Use these instead of FEAT_GLOBAL_ALPHA,
FEAT_GLOBAL_ALPHA_VID1 and FEAT_PRE_MULT_ALPHA in code.

Remove FEAT_GLOBAL_ALPHA_VID1 and FEAT_PRE_MULT_ALPHA which are no
longer used. FEAT_GLOBAL_ALPHA is still used to decide if the HW has
global alpha register.

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