First of all, it's only DP 1.2 output, not DP 1.4.
The graphics engine is in the processor as well. Whether it accesses DRAM using L4 tags in the L3 cache or the eDRAM controller should not matter (the DRAM remains the limiting factor). This doesn't have anything to do with the chipset, however, so I am not sure I understand exactly what you are asking.
The Intel Core i7-5775C processor is a Desktop processor and thus is designed for the LGA1150 socket, so no, BGA1364 is not okay. Further, Broadwell C-variant processors match with the Intel 9 Series chipsets (Z97, H97 and HM97), not the Intel 8 Series chipsets (Z87, H87, Q87, B85. H81).