drm/i915: use IS_JASPERLAKE()/IS_ELKHARTLAKE() instead of IS_PLATFORM()

Avoid direct IS_PLATFORM() usage when we have the platform helpers.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7ca5fabac6a6f5ac8e102b84d1e28502e79296f0.1712345787.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>