minigbm: rockchip: align NV12 planes to 64 bytes

minigbm: rockchip: align NV12 planes to 64 bytes

When running the mapped_texture_test with the Mali EGL implementation
compiled with debug symbols, Mali complains that the NV12 planes are
not sufficiently aligned (specifically, mali_cobj_surface.c line 1806).
Fix this.

BUG=b:63910596
TEST=run Youtube App video on Kevin, at fullscreen and at normal size

Change-Id: Ia204360c3aebe2bbd545ac7be0f6d095b0412f9d
Reviewed-on: https://chromium-review.googlesource.com/556540
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>