drm/amdgpu: remove set but not used variable 'pipe'

Fixes gcc '-Wunused-but-set-variable' warning:

rivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c: In function
‘amdgpu_gfx_graphics_queue_acquire’:
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:234:16: warning:
variable ‘pipe’ set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>