drm/amdgpu: Fix TLB invalidation request when using semaphore

Use a more meaningful variable name for the invalidation request
that is distinct from the tmp variable that gets overwritten when
acquiring the invalidation semaphore.

Fixes: 4ed8a037

 ("drm/amdgpu: invalidate mmhub semaphore workaround in gmc9/gmc10")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>