drm/amdkfd: Fix leftover errors and warnings

A bunch of errors and warnings are leftover KFD over the years, attempt
to fix the errors and most warnings reported by checkpatch tool. Still a
few warnings remain which may be false positives so ignore them for now.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>