drm/i915: split out i915_getparam.h from i915_drv.h

We already have the i915_getparam.c file.

Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/27f3af2298c3cdd3cb2839c2a9a52237248e087a.1641561552.git.jani.nikula@intel.com