drm/i915/gvt: remove unused variable gma_bottom in command parser

Remove unused variable gma_bottom in scan_workload() and scan_wa_ctx().
commit be1da707 ("drm/i915/gvt: vGPU command scanner") introduces
gma_bottom in several functions to calculate the size of the command
buffer. However, some of them are set but actually unused.

When compiling the code with ccflags -Wunused-but-set-variable, gcc
throws warnings.

Remove unused variables to avoid the gcc warnings. Tested via compiling
the code with ccflags -Wunused-but-set-variable.

Fixes: be1da707 ("drm/i915/gvt: vGPU command scanner")
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531020411.18987-1-zhi.a.wang@intel.com