i915_gem_do_execbuffer() holds the pages backing each relocation region for
various reasons while executing user commands. After these commands are
completed, the pages backing the relocation regions are unheld.

Since relocation regions do not have to be page aligned, the code in
validate_exec_list() allocates 2 extra page pointers in the array of
held pages populated by vm_fault_quick_hold_pages(). However, the cleanup
code that unheld the pages always assumed that only the buffer size /
PAGE_SIZE pages were used. This meant that non-page aligned buffers would
not unheld the last 1 or 2 pages in the list. Fix this by saving the
number of held pages returned by vm_fault_quick_hold_pages() for each
relocation region and using this count during cleanup.

Reviewed by: dumbbell, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D3965