gen7: fix GPU hang on WebGL texture-size test

gen7: fix GPU hang on WebGL texture-size test

When rendering to a texture with BaseLevel set, the miptree may be laid
out such that BaseLevel is in level 0 of the miptree (to avoid wasting
memory on unused levels between 0 and BaseLevel-1).  In that case, we
have to shift our render target's level down to the appropriate level of
the smaller miptree.

The WebGL test in combination with a meta code relating to
glGenerateMipmap also triggered a similar failure scenario.

This GPU hang regression was introduced by c754f7a8.

Bugzilla: http://bugs.freedesktop.org/show_bug.cgi?id=65324
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>