drm: omapdrm: Drop manual framebuffer pin handling

Since the removal of omap_plane_mode_set(), framebuffers are now pinned
exclusively through the plane .prepare_fb() and .cleanup_fb() operations
as the remaining callers of omap_plane_setup() don't modify the
framebuffer. Remove the manual pin/unpin infrastructure.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>