drm/amdgpu: explicitely set the AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS flag

Instead of having that in the amdgpu_bo_pin() function applied for all
pinned BOs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>