drm/gma500: remove helper function

We were getting build warning about:
drivers/gpu/drm/gma500/mdfld_dsi_output.c:407:2: warning: initialization
	from incompatible pointer type

The callback to dpms was pointing to a helper function which had a
return type of void, whereas the callback should point to a function
which has a return type of int.
On closer look it turned out that we do not need the helper function
since if we call drm_helper_connector_dpms() directly, the first check
that drm_helper_connector_dpms() does is: if (mode == connector->dpms)

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1454393155-13142-1-git-send-email-sudipm.mukherjee@gmail.com
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>