Hello Sunjib
Windows 10 IOT display driver for 64bit (imx8MQ included) doesn't support change display resolution yet. HDMI initialization (display) is performed in u-boot code (see imx8mq_evk.c file), display resolution is configured to the fix value 1280x720 60Hz. You can modify imx8mq_evk.c file in u-boot and HdLcdiMX8.c file in UEFI (mu_platform_nxp) on you own and rebuild u-boot & UEFI.
We plan to support of display resolution change in Windows 10 IOT display driver in future.
Best Regards
Jiri Rezler