drm/radeon: remove set but not used variable 'blocks'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/radeon/radeon_combios.c: In function radeon_combios_get_power_modes:
drivers/gpu/drm/radeon/radeon_combios.c:2638:10: warning: variable blocks set but not used [-Wunused-but-set-variable]

It is introduced by commit 56278a8e ("drm/radeon/kms:
pull power mode info from bios tables (v3)"), but never used,
so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>