drm/sis: Don't include <drm/drm_pci.h>

Including <drm/drm_pci.h> is unnecessary in most cases. Replace
these instances.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203100406.9674-11-tzimmermann@suse.de