drm/radeon: Only build fbdev if DRM_FBDEV_EMULATION is set

Make building fbdev emulation depend on DRM_FBDEV_EMULATION. Also
rename the source file to radeon_fbdev.c to align with other fbdev
files.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>