drm/msm: remove unused variable

A cleanup left behind an unused variable that we have to remove
in order to avoid this harmless warning:

drivers/gpu/drm/msm/adreno/a5xx_gpu.c: In function 'a5xx_zap_shader_init':
drivers/gpu/drm/msm/adreno/a5xx_gpu.c:493:19: error: unused variable 'a5xx_gpu' [-Werror=unused-variable]

Fixes: 8d6f0827 ("drm/msm: Remove uneeded platform dev members")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>