drm/i915/vblank: use display->platform.<platform> instead of IS_<PLATFORM>()

Switch to using the new display->platform.<platform> members for
platform identification in display code.

v2: Split out an unrelated hunk to a separate patch (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ed7295a7246bf00d8ae39f78c78dcc842c6939d9.1730144869.git.jani.nikula@intel.com


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