drm/radeon: cypress_dpm: Fix unused variable warning when CONFIG_ACPI=n

With CONFIG_ACPI=n the following build warning is seen:

drivers/gpu/drm/radeon/cypress_dpm.c:302:31: warning: unused variable 'eg_pi' [-Wunused-variable]

Protect eg_pi with CONFIG_ACPI.

Based on patch from: Fabio Estevam <fabio.estevam@freescale.com>
but doesn't mix allocation and code.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>