Fix lamp size allowing negative values.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57382 954f8c5b-7b00-dc11-b283-0030488c597c
Fix #35665: more CUDA issues with recent kernel changes, tested on sm_20, sm_21
and sm_30 cards, so hopefully it should all work now.

Also includes some warnings fixes related to nvcc compiler arguments, should make
no difference otherwise.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57383 954f8c5b-7b00-dc11-b283-0030488c597c