drm/amdgpu: add helper function for indirect reg access (v3)

Add helper function in order to remove RREG32/WREG32
in current pcie_rreg/wreg function for soc15 and
onwards adapters.
PCIE_INDEX/DATA pairs are used to access regsiters
outside of mmio bar in the helper functions.
The new helper functions help remove the recursion
of amdgpu_mm_rreg/wreg from pcie_rreg/wreg and
provide the oppotunity to centralize direct and
indirect access in a single function.

v2: Fixed typo and refine the comments

v3: Remove unnecessary volatile local variable

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>