drm/amdgpu: Replace HQD terminology with slots naming

The term "HQD" is CP-specific and doesn't
accurately describe the queue resources for other IP blocks like SDMA,
VCN, or VPE. This change:

1. Renames `num_hqds` to `num_slots` in amdgpu_kms.c to better reflect
   the generic nature of the resource counting
2. Updates the UAPI struct member from `userq_num_hqds` to `userq_num_slots`
3. Maintains the same functionality while using more appropriate terminology

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>