amdgpu/pm: Optimize emit_clock_levels for arcturus - part 1

 Use variables to remove the multiple nested ternary expressions and
 improve readability. This will help to optimize the code duplication
 in the switch statement
 Also Changed:
  Modify function arcturus_get_clk_table to void function as it
    always returns 0
  Use const string "attempt_string" to cut down on repetition

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>