drm/amdkfd: Simplify the mmap offset related bit operations

The new code uses straightforward bit shifts and thus has better readability.

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