drm: rockchip: remove rockchip_drm_framebuffer_init() function

The function rockchip_drm_framebuffer_init() was in use
in the rockchip_drm_fbdev.c file, but that is now replaced
by a generic fbdev setup. Reduce the image size by
removing the rockchip_drm_framebuffer_init() and sub function
rockchip_fb_alloc() and cleanup the rockchip_drm_fb.h header file.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: John Keeping <john@metanate.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/ebe91504-c5df-99e4-635f-832218584051@gmail.com