mesa: set clamp bit in glGetTexImage for GL_UNSIGNED_NORMALIZED

mesa: set clamp bit in glGetTexImage for GL_UNSIGNED_NORMALIZED

Color clamping should be enabled in glGetTexImage if texture dataType is
GL_UNSIGNED_NORMALIZED and format is GL_LUMINANCE or GL_LUMINANCE_ALPHA

Fixes 2 Intel oglconform test cases: pxconv-gettex and pxtrans-gettex
https://bugs.freedesktop.org/show_bug.cgi?id=40864

NOTE: This is a candidate for the 8.0 branch

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit 5665b5cc31da70e833f80e7a17bfa034d2f7ba44)