drm/i915/pfit: Reject pfit downscaling for GMCH platforms

drm/i915/pfit: Reject pfit downscaling for GMCH platforms

Gen2/3 pfit doesn't support downscaling at all, so reject it.

On i965+ downscaling is supported by the hardware (max scale
factor < 2.0), but as downscaling increases the effective
pixel rate we can't safely allow it unless
intel_crtc_compute_pixel_rate() gets fixed. Probably the
best solution would be to calculate (at least an
apporiximate) pfit destination window and use
ilk_pipe_pixel_rate() for all platforms. For now reject
downscaling on all gmch platforms.

The intel ddx has a similar check for this in userspace,
modesetting ddx does not. And presumably wayland compositors
also do not make such assumptions in userspace.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016143134.26903-4-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>