drm/amd: Do not include <linux/export.h> when unused

Fix the following compile time warning when building with W=1:

warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>