drm/radeon: use lower_32_bits where appropriate

Replace occurrences of "v & 0xffffffff" with lower_32_bits(v)
when it's next to an upper_32_bits(v). Also remove unnecessary
"upper_32_bits(v) & 0xffffffff" code snippets.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>