i965: Configure bufmgr debug options from intel_screen.c

i965: Configure bufmgr debug options from intel_screen.c

We need the debug flag parsing and INTEL_DEBUG in the compiler, but we
don't want the dependency on bufmgr (libdrm_intel) in there. Move to
intel_screen.c.

There are now only two lines left in brw_process_intel_debug_variable(),
but we keep it in intel_debug.h to avoid having to expose
'debug_control' as a global variable.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>