drm/radeon/btc_dpm: Clean up errors in btc_dpm.c

Fix the following errors reported by checkpatch:

ERROR: that open brace { should be on the previous line
ERROR: space required before the open parenthesis '('
ERROR: spaces required around that '+=' (ctx:VxV)
ERROR: spaces required around that '=' (ctx:VxW)
Signed-off-by: GuoHua Chen <chenguohua_716@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>