media: sti: fix obj-$(config) targets

The right thing to do is to add a new object to the building
system when a certain config option is selected, and *not*
override them.

So, fix obj-$(config) logic at sti makefiles, using "+=",
instead of ":=".
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/platform/sti/bdisp/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_VIDEO_STI_BDISP) := bdisp.o
+obj-$(CONFIG_VIDEO_STI_BDISP) += bdisp.o
 
 bdisp-objs := bdisp-v4l2.o bdisp-hw.o bdisp-debug.o

drivers/media/platform/sti/delta/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_VIDEO_STI_DELTA_DRIVER) := st-delta.o
+obj-$(CONFIG_VIDEO_STI_DELTA_DRIVER) += st-delta.o
 st-delta-y := delta-v4l2.o delta-mem.o delta-ipc.o delta-debug.o
 
 # MJPEG support

drivers/media/platform/sti/hva/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_VIDEO_STI_HVA) := st-hva.o
+obj-$(CONFIG_VIDEO_STI_HVA) += st-hva.o
 st-hva-y := hva-v4l2.o hva-hw.o hva-mem.o hva-h264.o
 st-hva-$(CONFIG_VIDEO_STI_HVA_DEBUGFS) += hva-debugfs.o