drm/i915: Move load time gem_load_init earlier

The only steps requiring device access is the fence and swizzling
initialization, so split these out keeping them in their current place
and move the rest of init steps earlier.

v2-v3:
- unchanged
v4:
- move call to i915_gem_detect_bit_6_swizzle() to
  i915_gem_load_init_fences() and preserve the original order of
  the detection of HW fence capailities wrt. swizzling (Chris)

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458132843-21860-1-git-send-email-imre.deak@intel.com