drm/vkms: check plane_composer->map[0] before using it

drm/vkms: check plane_composer->map[0] before using it

Fix a copypasta error. The caller of compose_plane() already checks
primary_composer->map. In contrast, plane_composer->map is never
verified here before handling.

Fixes: 7938f4218168 ("dma-buf-map: Rename to iosys-map")
Reviewed-by: André Almeida <andrealmeid@riseup.net>
Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220415111300.61013-2-tales.aparecida@gmail.com