OMAPDSS/OMAP_VOUT: Fix incorrect OMAP3-alpha compatibility setting

On OMAP3, in order to enable alpha blending for LCD and TV managers, we needed
to set LCDALPHABLENDERENABLE/TVALPHABLENDERENABLE bits in DISPC_CONFIG. On
OMAP4, alpha blending is always enabled by default, if the above bits are set,
we switch to an OMAP3 compatibility mode where the zorder values in the pipeline
attribute registers are ignored and a fixed priority is configured.

Rename the manager_info member "alpha_enabled" to "partial_alpha_enabled" for
more clarity. Introduce two dss_features FEAT_ALPHA_FIXED_ZORDER and
FEAT_ALPHA_FREE_ZORDER which represent OMAP3-alpha compatibility mode and OMAP4
alpha mode respectively. Introduce an overlay cap for ZORDER. The DSS2 user is
expected to check for the ZORDER cap, if an overlay doesn't have this cap, the
user is expected to set the parameter partial_alpha_enabled. If the overlay has
ZORDER cap, the DSS2 user can assume that alpha blending is already enabled.

Don't support OMAP3 compatibility mode for now. Trying to read/write to
alpha_blending_enabled sysfs attribute issues a warning for OMAP4 and does not
set the LCDALPHABLENDERENABLE/TVALPHABLENDERENABLE bits.

Change alpha_enabled to partial_alpha_enabled in the omap_vout driver. Use
overlay cap "OMAP_DSS_OVL_CAP_GLOBAL_ALPHA" to check if overlay supports alpha
blending or not. Replace this with checks for VIDEO1 pipeline.

Cc: linux-media@vger.kernel.org
Cc: Lajos Molnar <molnar@ti.com>
Signed-off-by: Archit Taneja <archit@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>