i965: Don't forget to subtract mt->first_level in minify calls.

i965: Don't forget to subtract mt->first_level in minify calls.

This fixes fbo-clear-formats GL_ARB_depth_texture on Ironlake, which
regressed since commit f128bcc7c293013f4b44e4b661638333de0077c2
("i965: Drop mt->levels[].width/height.")  intel_miptree_copy_slice was
calling minify(.., 7) on a 2x2 texture with mt->first_level == 7.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75292
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>