drm/amdkfd: Use prange->update_list head for remove_list

The remove_list head was only used for keeping track of existing ranges
that are to be removed from the svms->list. The update_list was used for
new or existing ranges that need updated attributes. These two cases are
mutually exclusive (i.e. the same range will never be on both lists).
Therefore we can use the update_list head to track the remove_list and
save another 16 bytes in the svm_range struct.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>