i965: Add support for 16-bit unorm L, A, and I textures.

i965: Add support for 16-bit unorm L, A, and I textures.

While not required by any particular spec version, mplayer was asking
for L16 and hoping for actual L16 without checking.  The 8 bits
allocated led to 10-bit planar video data stored in the lower 10 bits
giving only 2 bits of precision in video.  While it was an amusing
effect, give them what they actually wanted instead.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41461

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>