drm/amdkfd: Check for NULL return values

drm/amdkfd: Check for NULL return values

This patch fixes issues when the code moves forward with a potential
NULL pointer, without checking.
Removed one redundant NULL check for a function parameter. This check
is already done in the only caller.

Signed-off-by: Andrew Martin <andrew.martin@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>