video: omapfb2: Make standard and custom DSI command mode panel driver mutually exclusive

Standard DRM panel driver for DSI command mode panel used by omapfb2 is also
available now. Just like the other panels its module name clashes with the
module from drivers/video/fbdev/omap2/omapfb/displays, part of the deprecated
omapfb2 fbdev driver. As omapfb2 can only be compiled when the omapdrm driver
is disabled, and the DRM panel drivers are useless in that case, make the
omapfb2 panel depend on the standard DRM panels being disabled to fix
the name clash.

Fixes: cf64148a ("drm/panel: Move OMAP's DSI command mode panel driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108112441.14609-1-sebastian.reichel@collabora.com