drm/amdkfd: Change x==NULL/false references to !x

Upstream prefers the !x notation to x==NULL or x==false. Along those lines
change the ==true or !=NULL references as well. Also make the references
to !x the same, excluding () for readability.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>