drm/amdgpu: Remove unused variable

Remove unused space_needed variable.

Fixes: 453f617a ("drm/amdgpu: Resize BAR0 to the maximum available size, even if it doesn't cover VRAM")
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/413807/