drm/amdkfd: use vma_lookup() instead of find_vma()

Using vma_lookup() verifies the start address is contained in the found
vma.  This results in easier to read the code.

Signed-off-by: Deming Wang <wangdeming@inspur.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>