With kernel 3.15.5 X does not start. I get an error about i915 module failed to load. modprobe i195 results in "could not insert i915: no such device". Everything runs fine with kernel 3.15.2.
Hmm, you may want to check your module installation is all right. Please attach dmesg with drm.debug=0xe module parameter set for the failing case.
Sorry for the late response. I just updated to kernel 3.16.3. Looks like everything is okay again. Let me know if you need any further information.
Since 3.15 is EOL and 3.16 works, I'll just close this without further investigation. Thanks for the report, and please do reopen if the problem reappears.