drm/omap: Clean up deadcode functions

dispc_enable_fifomerge() last use was removed by 2012's
commit 85099f11 ("Revert "OMAPDSS: APPLY: add fifo merge support
funcs"")

dispc_has_writeback(), dispc_wb_get_framedone_irq(), dispc_wb_go(),
dispc_wb_go_busy() and dispc_wb_setup() were changed from statics
to public symbols and unwired from a structure by 2020's
commit dac62bca ("drm/omap: remove dispc_ops")
but didn't have any users.

dispc_mgr_get_clock_div() got renamed from dispc_get_clock_div()
and it's last use was removed in 2011 by commit
42c9dee8 ("OMAP: DSS2: Remove FB_OMAP_BOOTLOADER_INIT support")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241026220010.93773-1-linux@treblig.org