mesa: Fix Get(GL_TRANSPOSE_CURRENT_MATRIX_ARB) to transpose

This was just returning the same value as GL_CURRENT_MATRIX_ARB.
Spotted while investigating something else in apitrace.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 2b4fe85f)