drm/tilcdc: Add dummy primary plane implementation

Add dummy primary plane implementation. LCDC does not really have
planes, only simple framebuffer that is mandatory. This primary plane
implementation has the necessary checks for implementing simple
framebuffer trough DRM plane abstraction. For setting the actual
framebuffer the implementation relies on a CRTC side function.

Signed-off-by: Jyri Sarha <jsarha@ti.com>