drm/i915/display: Prevent u64 underflow in intel_fbc_stolen_end

drm/i915/display: Prevent u64 underflow in intel_fbc_stolen_end

Static analysis reveals a potential integer underflow in
intel_fbc_stolen_end.  This can apparently occur if
intel_parent_stolen_area_size returns zero (or, theoretically, any value
less than 2^23), as 2^23 is subtracted from the return value and stored
in a u64.  While this doesn't appear to cause any issues due to the use
of the min() function to clamp the return values from the
intel_fbc_stolen_end function, it would be best practice to avoid
undeflowing values like this on principle.  So, rework the function to
prevent the underflow from occurring.  Note that the underflow at
present would result in the value of intel_fbc_cfb_base_max being
returned at the end of intel_fbc_stolen_end, so just return that if the
value of intel_parent_stolen_area_size is too small.

While we're here, fix the other comments here and modify the execution
path for readability.

v2: (Jani)
- Fix the comments in intel_fbc_stolen_end
- Use check_sub_overflow
- Remove macro that mirrors SZ_8M, as it is now only referenced once
- Misc. formatting fixes

Fixes: a9da512b3ed7 ("drm/i915: avoid the last 8mb of stolen on BDW/SKL")
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20260107162935.8123-2-jonathan.cavitt@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
(cherry picked from commit 6695dc279820a50cb20ecd8b5250e05234dac780)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>