iris: glClear with FBO imported from DMA-BUF doesn't work

System information

System:    Host: localhost.localdomain Kernel: 5.6.2-301.fc32.x86_64 x86_64 bits: 64 compiler: gcc v: 10.0.1 
           Console: N/A Distro: Fedora release 32 (Thirty Two) 
CPU:       Topology: Quad Core model: Intel Core i5-8350U bits: 64 type: MT MCP arch: Kaby Lake rev: A 
           L1 cache: 256 KiB L2 cache: 6144 KiB L3 cache: 6000 KiB 
           flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 30399 
           Speed: 800 MHz min/max: 400/3600 MHz Core speeds (MHz): 1: 800 2: 801 3: 800 4: 801 5: 800 6: 800 7: 800 
           8: 800 
Graphics:  Device-1: Intel UHD Graphics 620 vendor: Lenovo driver: i915 v: kernel bus ID: 00:02.0 
           chip ID: 8086:5917 
           Device-2: IMC Networks Integrated Camera type: USB driver: uvcvideo bus ID: 1-8:2 chip ID: 13d3:584b 
           Display: server: X.org 1.20.8 driver: modesetting FAILED: fbdev,intel,vesa 
           Message: Advanced graphics data unavailable for root. 

Description

1. Create a DMA-BUF with gbm_bo_create
2. Import it into EGL as an EGLImage
3. Create a RBO backed by the EGLImage, then create an FBO from that RBO
4. Bind to the FBO, set the viewport, and call glClearColor+glClear to paint the buffer red
5. Submit the DMA-BUF to KMS for display

Expected: the output is red. However with iris the output is black or scrambled. This only affects glClear, drawing textures works fine.

All of this can be reproduced by running examples/simple in a TTY from this wlroots branch: https://github.com/swaywm/wlroots/pull/2240

Works fine with i965 (and other drivers).