mesa/main/texgetimage: use master conversion function in get_tex_rgba_uncompressed()

Updates get_tex_rgba_uncompressed() to use always use the master
conversion function.

It fixes 22 tests in piglit for i965 driver (SandyBridge)

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>