backend-drm: simplify compile time array copy

In drm_fb_get_from_dmabuf() we have some compile time array copies, and
multiple static_assert() calls are needed (for safety). This makes the
code unpleasant to read.

Add ARRAY_COPY macro, to simplify the code.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>