drm/amdkfd: Fix memory leak in create_process failure

Fix memory leak due to a leaked mmget reference on an error handling
code path that is triggered when attempting to create KFD processes
while a GPU reset is in progress.

Fixes: 0ab2d753 ("drm/amdkfd: prepare per-process debug enable and disable")
CC: Xiaogang Chen <xiaogang.chen@amd.com>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Tested-by: Harish Kasiviswanthan <Harish.Kasiviswanthan@amd.com>
Reviewed-by: Mukul Joshi <mukul.joshi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org