Numerous comments, clean-ups, and refactorings for the DRI portion of
the MGA driver.

- Use mga_drm.h instead of mga_common.h.  Due to this, all the
  MGA-specific DRM names were changed to generic versions (i.e.,
  drmMGALock became drm_lock_t).
  
- Merge MGADRIMapInit into MGADRIAGPInit and rename the resulting function
  MGADRIBootstrapDMA.

- Eliminate several unnecessary drmMap calls.  The server has no need to
  map MMIO space, primary DMA buffer space, secondary DMA buffer space,
  WARP microcode space, or the DRM status page.

- Clarified which portions of the client / server DRI interface are no
  longer used.

- Removed some unused fields from MGADRIServerPrivateRec.

- Bump MGA_PATCHLEVEL.