drm: omap: remove obsolete selection of OMAP2_DSS in config DRM_OMAP

Commit 55b68fb8 ("drm/omap: squash omapdrm sub-modules into one")
removes the config OMAP2_DSS in ./drivers/gpu/drm/omapdrm/dss/Kconfig,
while moving the other configs into./drivers/gpu/drm/omapdrm/Kconfig, but
misses to remove an obsolete selection of OMAP2_DSS in config DRM_OMAP.

Hence, ./scripts/checkkconfigsymbols.py warns:

OMAP2_DSS
Referencing files: drivers/gpu/drm/omapdrm/Kconfig

Remove this reference in an obsolete selection.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819112253.16484-6-lukas.bulwahn@gmail.com