drm: Include <asm/agp.h> iff CONFIG_AGP is set

drm: Include <asm/agp.h> iff CONFIG_AGP is set

Platforms without AGP don't provide the AGP header. Only include it
if CONFIG_AGP is set.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: 04dfe19a5ed6 ("drm: Mark AGP implementation and ioctls as legacy")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Jason Yan <yanaijie@huawei.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210510145218.10322-1-tzimmermann@suse.de