drm/amdkfd: clean up the code to free hmm_range

drm/amdkfd: clean up the code to free hmm_range

a. hmm_range is either NULL or a valid pointer so we
do not need to set range to NULL ever.

b. keep the hmm_range_free in the end irrespective of
the other conditions to avoid some additional checks
and also avoid double free issue.

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