mesa/texformat: Handle GL_RGBA + GL_UNSIGNED_SHORT_5_5_5_1

mesa/texformat: Handle GL_RGBA + GL_UNSIGNED_SHORT_5_5_5_1

_mesa_choose_tex_format() already handles GL_RGBA + GL_UNSIGNED_SHORT_1_5_5_5_REV
by converting it to MESA_FORMAT_B5G5R5A1_UNORM. Teach it do the same for
the non-reversed type. Otherwise, the switch's fallthrough converts it
to an 8888 format, which has incompatible precision in the alpha
channel.

Patch 2/2 to fix dEQP-EGL.functional.image.modify.tex_rgb5_a1_tex_subimage_rgba8
on Intel.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99185
Cc: Haixia Shi <hshi@chromium.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>