drm: mxsfb: Rename mxsfb_crtc.c to mxsfb_kms.c

The mxsfb_crtc.c file doesn't handle just the CRTC, but also the other
KMS objects. Rename it accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-11-laurent.pinchart@ideasonboard.com