iris: call brw_process_intel_debug_variable() earlier
We're currently only calling it after creating the screen and the bufmgr. There are a few cases where Iris checks for the DEBUG_BUFMGR bit before we call brw_process_intel_debug_variable(), which means intel_debug is 0 and so we don't run the debug code. Today, these are all related to the creation of the workaround bo and its mmap.
I found this in a custom branch after I converted to INTEL_DEBUG an environment variable that I had.
Cc: @jljusten