OMAPDSS: DPI: Remove cpu_is_xxxx checks

The OMAP3 checks have been removed and replaced by a dss feature
FEAT_DPI_USES_VDDS_DSI for cleaner implementation. The patches
"OMAP: DSS2: enable VDDS_DSI when using DPI" (8a2cfea8) by Tomi Valkeinen
<tomi.valkeinen@nokia.com> and "ARM: omap: fix oops in
drivers/video/omap2/dss/dpi.c" (40410715) by Russell King
<rmk+kernel@arm.linux.org.uk> had introduced these checks. As it is evident
from these patches a dependency exists for some DSS pins on VDDS_DSI which is
better shown by dss feature FEAT_DPI_USES_VDDS_DSI.

Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>