fbdev: omapfb: Remove writeback deadcode

Value of enum parameter 'plane' is initialized in dss_init_overlays and
cannot take the value OMAP_DSS_WB. Function dispc_ovl_setup_common could
be called with this value of parameter only from dispc_wb_setup, which has
never been used and has been removed in commit 4f55bb03
("omapfb: Remove unused writeback code"). The code in the if-branch is
unreachable.

Remove unreachable branch.

Found by Linux Verification Center (linuxtesting.org) with SVACE static
analysis tool.

Signed-off-by: Leonid Arapov <arapovl839@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>