drm/i915: Restore pci_enable_busmaster() call in the init path

This fixes a GPU hang on i945GM.

While here, merge some minor fixes to DRM core and i915:

• Remove obsolete drm_agp_*_memory() prototypes
• Fix comment in drm_fops.c (outisde -> outside)
• Fix some formatting issues in drm_stub.c (spaces -> tabs)

Approved by: re (marius)
MFC of: r288653, r288952, r293851
Submitted by: <s3erios@gmail.com>
Differential Revision: https://reviews.freebsd.org/D3413