drm2: a few minor fixes after r280183

• Remove obsolete drm_agp_*_memory() prototypes.
• Fix comment in drm_fops.c (outisde -> outside).
• Fix some formatting issues in drm_stub.c (spaces -> tabs).
• Add missing case statement (gen == 3) in intel_gpu_reset().
• Restore pci_enable_busmaster() call in the init path (fixes gpu hang on i945GM).
• Replace M_WAITOK with M_NOWAIT when the return value of malloc is checked (may be incorrect).

Submitted by: <s3erios@gmail.com>
Reviewed by: dumbbell
Approved by: dumbbell
Differential Revision: https://reviews.freebsd.org/D3413