[NFC][AMDGPU] Reduce includes dependencies.

1. Splitted out some parts of R600 target to separate modules/headers.
2. Reduced some include lists in headers.
3. Found and fixed issue with override `GCNTargetMachine::getSubtargetImpl()`
   and `R600TargetMachine::getSubtargetImpl()` had different return value type
   than base class.
4. Minor forward declarations cleanup.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D108596