fbdev/leo: Use initializer macro for struct fb_ops

Initialize struct fb_ops to the correct default values with the
macro FB_DEFAULT_SBUS_OPS(). Rename the ioctl and mmap callbacks
to use the infix _sbusfb_. This makes them fit the SBUS helpers'
naming pattern. Also make the driver depend on FB_SBUS_HELPERS,
which selects the correct modules.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230806120926.5368-10-tzimmermann@suse.de