drm/i915/gt: Remove bogus comment on IVB_FBC_RT_BASE_UPPER

The comment on the parameter being 0 to avoid the read back doesn't
apply as this is not a call to wa_add(), but rather to
wa_write_clr_set(). So, this register is actually checked and it's
according to the Bspec that the register is RW, not RO.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630203509.1635216-7-lucas.demarchi@intel.com