OK, found it, problem is in the board file: boards/base/STM32F746-Discovery/board_STM32LTDC.h
1) Line 139 for PK0 MODER used is GPIO_MODER_MODER0_0 but should be GPIO_MODER_MODER0_1
2) Line 184 for PG12 AFRH is set to 14 (LCD_B1), but needs to be 9 (LCD_B4), LCD_B1 we have on PJ13!
Now there is only one thing left, HardFault when enabling RGB888 mode in drivers config file, I overcome this by setting LTDC_PIXELFORMAT to LTDC_FMT_ARGB8888, LTDC_PIXELBYTES to 4 and LTDC_PIXELBITS to 32, also, in DMA2D init if RGB888 is selected, I set DMA2D to ARGB8888. Yes I know, this is not the right way, but, at least, it doesn't HardFault, HardFault error was unaligned memory access, 24 bits on 32bit cpu?!? Dunno why, I am not so smart in all this, and last line in gdisp_lld_STM32LTDC.c executed before HardFault is 277: PIXEL_ADDR(g, pos)[0] = gdispColor2Native(g->p.color);
Anyways, now I can continue do what I wanted to do on F7-Disco heh and you have all the info needed to fix all those problems, case closed!