gpu/drm: cleanup coding style a bit

Remove first assignment to info which is meaningless.
Put the width and higth check first.
This change is to make the code a bit readable.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907123129.27905-1-bernard@vivo.com