amdgpu/pm: set pp_dpm_dcefclk to readonly on NAVI10 and newer gpus

v2 : change condition to apply to all chips after NAVI10

Writing to dcefclk causes the gpu to become unresponsive, and requires a reboot.
Patch prevents user from successfully writing to file pp_dpm_dcefclk on parts
NAVI10 and newer, and gives better user feedback that this operation is not allowed.

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>