drm/amdgpu: optimize VRAM allocation when using drm buddy

Since the VRAM manager changed from drm mm to drm buddy. It's
not necessary to allocate 2MB aligned VRAM for more than 2MB
unaligned size, and then do trim. This method improves the
allocation efficiency and reduces memory fragmentation.

v2: Correct the remainder operation

Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>