drm/fourcc: Add 16 bpc fixed point framebuffer formats.

These are 16 bits per color channel unsigned normalized formats.
They are supported by at least AMD display hw, and suitable for
direct scanout of Vulkan swapchain images in the format
VK_FORMAT_R16G16B16A16_UNORM.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>