drm/via: fix assignment in if condition

Fix check_patch.pl warning:
ERROR: do not use assignment in if condition
+                       if ((HALCYON_HEADER2 == (cmd = *buf)) &&

ERROR: do not use assignment in if condition
+                       if (HALCYON_HEADER2 == (cmd = *buf))

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201118114021.105502-1-bernard@vivo.com