drm/tegra: Hide fbdev support behind config option

Only build tegra's fbdev emulation if CONFIG_DRM_FBDEV_EMULATION
has been enabled. As part of this change, move the code into its
own source file. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>