drm/i915: Rename "_load"/"_unload" to match PCI entry points

Current names of i915_driver_load/unload() functions originate in
legacy DRM stubs.  Reduce nomenclature ambiguity by renaming them to
match their current use as helpers called from PCI entry points.

Suggested by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712112429.740-3-janusz.krzysztofik@linux.intel.com