drm/amd: Include <linux/export.h> when needed

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

warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

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>