drm/i915/vgpu: improve vgpu abstractions

Add intel_vgpu_register() abstraction, rename i915_detect_vgpu() to
intel_vgpu_detect() to match other function naming, un-inline
intel_vgpu_active(), intel_vgpu_has_full_ppgtt() and
intel_vgpu_has_huge_gtt() to reduce header interdependencies.

The i915_vgpu.[ch] filename and intel_vgpu_ prefix discrepancy remains.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227144408.24345-1-jani.nikula@intel.com