I can confirm I am having the same issue for the last few weeks, with the same make.log error messages, resulting in no graphics upon boot. Only solution I've found so far is to remove the 5.14 kernel. All other packages are fully updated to Sid.
- Bug#994892: evdi-dkms: fails to build for kernel 5.14.0-1 RĂ©mi Letot
- Bug#994892: evdi-dkms: fails to build for kernel 5.14.... Justin Searle
- Bug#994892: evdi-dkms: fails to build for kernel 5... Hanno Stock
- Bug#994892: evdi-dkms: fails to build for kern... Michael Crusoe