It happens because you lack the necessary patches to the framebuffer.
There are some patches for HDMI-audio in my config.plist examples... In particular the variants with "(12)".
Although you may not need the HDMI-audio changes as your laptop has DP.
Let me know how it goes...
https://github.com/RehabMan/OS-X-Clover-Laptop-Config
For example two patches (patches specific to 0xa260006 ports, as that is the ig-platform-id you're using)...
Code:
HDMI-audio, port 0105(12), 0x0a260005 0x0a260006, 0x0d220003, 0x0a2e000a, 0x0a26000a
find: <01050900 00040000 87000000>
repl: <01051200 00080000 87000000>
and
Code:
HDMI-audio, port 0204(12), 0x0a260005 0x0a260006
find: <02040900 00040000 87000000>
repl: <02041200 00080000 87000000>
You can take the HDMI part out by changing the patches slightly (to keep DP connector type 00 04 00 00)...
Code:
HDMI-audio, port 0105(12), 0x0a260005 0x0a260006, 0x0d220003, 0x0a2e000a, 0x0a26000a
find: <01050900 00040000 87000000>
repl: <01051200 00040000 87000000>
and
Code:
HDMI-audio, port 0204(12), 0x0a260005 0x0a260006
find: <02040900 00040000 87000000>
repl: <02041200 00040000 87000000>