drm: drm_atomic_uapi.c: Use tabs for code indents

Remove space and use tabs for indent the code to follow the
Linux kernel coding conventions.
Problem found by checkpatch.

Signed-off-by: Beatriz Martins de Carvalho <martinsdecarvalhobeatriz@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/dc8286f5590fff609f924845fb622dd5f962a11b.1618828127.git.martinsdecarvalhobeatriz@gmail.com