drm/amdgpu: Add support for SRBM selection v3

Also remove code duplication in write and read regs functions.
This also fixes potential missing unlock in amdgpu_debugfs_regs_write
in case get_user would fail.

v2: Add SRBM mutex locking.
v3: Fix TO counter and fix comment location.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>