drm/amd/powerplay: use ARRAY_SIZE() to calculate array size.

It is preferred to use ARRAY_SIZE() for size calculation, instead
using sizeof(array)/sizeof(*array). It makes the code more readable.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>