drm/imx: Remove local fbdev emulation Kconfig option

drm/imx: Remove local fbdev emulation Kconfig option

DRM_IMX_FB_HELPER config is currently used to enable/disable fbdev
emulation for the imx kms driver.

Remove this local config option and use the top level DRM_FBDEV_EMULATION
config option where applicable. Using this config lets us also prevent
wrapping around drm_fb_helper_* calls with #ifdefs in certain places.

Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1445933459-5249-2-git-send-email-architt@codeaurora.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>