I have a WaveShare 3.5in GPIO RPi LCD(A),According wiki: HCL:Raspberry Pi3 LCD
follow steps modified extraconfig.txt added dtoverlay=rpi-display ,but at
reboot,the touchscreen is Blank screen only,but backlight is on.
Run dmesg|grep screen output:
suse-pi:~ # dmesg |grep screen
[   28.907749] ads7846 spi0.1: touchscreen, irq 79
[   28.908498] input: ADS7846 Touchscreen as
/devices/platform/soc/3f204000.spi/spi_master/spi0/spi0.1/input/input0

It show kernel is identified the touchscreen,but no display output.
output: dmesg|grep gpu
[   29.844885] vc4-drm soc:gpu: bound 3f902000.hdmi (ops vc4_hdmi_ops [vc4])
[   29.845227] vc4-drm soc:gpu: bound 3f806000.vec (ops vc4_vec_ops [vc4])
[   29.845467] vc4-drm soc:gpu: bound 3f004000.txp (ops vc4_txp_ops [vc4])
[   29.845586] vc4-drm soc:gpu: bound 3f400000.hvs (ops vc4_hvs_ops [vc4])
[   29.846304] vc4-drm soc:gpu: bound 3f206000.pixelvalve (ops vc4_crtc_ops
[vc4])
[   29.846992] vc4-drm soc:gpu: bound 3f207000.pixelvalve (ops vc4_crtc_ops
[vc4])
[   29.847685] vc4-drm soc:gpu: bound 3f807000.pixelvalve (ops vc4_crtc_ops
[vc4])
[   29.871458] vc4-drm soc:gpu: bound 3fc00000.v3d (ops vc4_v3d_ops [vc4])
[   29.874539] [drm] Initialized vc4 0.0.0 20140616 for soc:gpu on minor 0
[   29.939148] vc4-drm soc:gpu: fb0: DRM emulated frame buffer device