The DragonBoard 410c does not generate the Hot Plug Detect signal. The DragonBoard generates a +5V signal that is sent to the monitor. The Monitor then responds by asserting the Hop Plug detect signal. The hot plug detect signal is generated by your monitor. The monitor should not generate the Hot Plug detect signal until it is ready.
I think you need to attach an Oscilloscope to the +5V, HPD, and DDC signals to determine why your monitor is generating a Hot Plug Detect signal before it is ready.
Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.