fbdev/sa1100fb: Remove even more dead code

This function lost its only call site as part of
earlier dead code removal, so remove it as well:

drivers/video/fbdev/sa1100fb.c:975:21: error: unused function 'sa1100fb_min_dma_period' [-Werror,-Wunused-function]

Fixes: 390e5de1

 ("fbdev/sa1100fb: Remove dead code")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906151307.1127187-1-arnd@arndb.de