I have already put quite a time into getting ugfx (2.7) and stm32f746 to work with chibios (stable_16.1.x branch).
This bare metal version works fine:
Chibios ugfx for stm32f746 is broken because it does not include needed stm32cube headers. So copy STM32f7xx_HAL_Driver from BareMetal project to your project dir and for quick hack modify ugfx/boards/base/STM32F746-Discovery/board.mk to include stuff from Cube HAL:
GFXINC += $(STMHAL)/Inc
Also modify your own Makefile that should be copied from ugfx/boards/base/STM32F746-Discovery/example_chibios3
#Special - Required for the drivers for this discovery board.
STMHAL = STM32F7xx_HAL_Driver
After that dependencies are resolved and project can be built, BUT it still does not work and is just showing blank screen with backlight on. So there is still something fishy.
