drm/imx: compile imx directory by default

Currently the drm/imx/ directory is compiled only if DRM_IMX is set. Adding a
new IMX related IP in the same directory would need DRM_IMX to be set, which would
bring in also IPUv3 core driver...

The current patch would allow adding new IPs in the imx/ directory without needing
to set DRM_IMX.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200731081836.3048-2-laurentiu.palcu@oss.nxp.com