drm/i915/gt: Drop read from GEN8_L3CNTLREG in ICL workaround

Now that non-masked registers are already read before programming the
context reads, the additional read became redudant, so remove it.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630203509.1635216-5-lucas.demarchi@intel.com