i915_gem: Assert page array size.

Let's detect the bug of sg_npgs failing to match
obj->base.size/PAGE_SIZE earlier.

Related to PR kern/57833.

XXX pullup-10

i915: Zero the kernel's ptes before freeing the va.

drm: Another pass over i915 and some supporting logic.

This makes a shim around sg_table, which essentially represents two
things:

1. an array of pages (roughly corresponding to bus_dma_segment_t[])
2. an array of DMA addresses stored in a bus_dmamap_t

Both parts are optional; different parts of i915 use sg_tables to
pass around one or both of the two parts. This helps to reduce the
ifdefs by quite a bit, although it's not always clear which part of
an sg_table any particular interface is actually using which is why I
was reluctant to do this before.

Sync i915_gem_fault. Hack up the rest of i915_gem.c enough to build.

First pass at i915, far from complete.

HEAVILY MODIFIED IN MERGE - maya

Merge linux-drm-v5-6-rc3

branches: 1.1.1;
Initial revision