drm/i915/display: use clk_off name to avoid double negation

Instead of "ungated" use the same name for the variable as the bitfield,
making it clearer what's the intent of the checks.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217230529.25092-2-lucas.demarchi@intel.com