drm/tilcdc: remove unnecessary state->fb check

tilcdc_plane_atomic_check() exits if state->fb == NULL, so no need to
check it again later.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429104234.18910-2-tomi.valkeinen@ti.com


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