drm/amdgpu: clean up amdgpu hmm range functions

drm/amdgpu: clean up amdgpu hmm range functions

Clean up the amdgpu hmm range functions for clearer
definition of each.

a. Split amdgpu_ttm_tt_get_user_pages_done into two:
   1. amdgpu_hmm_range_valid: To check if the user pages
      are valid and update seq num
   2. amdgpu_hmm_range_free: Clean up the hmm range
      and pfn memory.

b. amdgpu_ttm_tt_get_user_pages_done and
   amdgpu_ttm_tt_discard_user_pages are similar function so remove
   discard and directly use amdgpu_hmm_range_free to clean up the
   hmm range and pfn memory.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>