DRM: armada: fix missing DRM_KMS_FB_HELPER select

Commit 92b6f89f6b8f (drm: Add separate Kconfig option for fbdev helpers)
happened in parallel with the inclusion of Armada DRM into mainline,
and so missed this update.  Add the necessary select statement to avoid
build errors.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>