If a uniform was already marked as dirty, and the call to
setMatrixUniform wasn't updating the data, the flag was flipped
to false, causing the descriptor set never to be initialized.

This enables these tests:
dEQP-GLES2.functional.shaders.conversions.scalar_to_matrix.*

Bug: angleproject:2583

Change-Id: I0bcb95cc66951e0ed347af5d874178c403a1c08a
Reviewed-on: https://chromium-review.googlesource.com/1085228Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>