drm: radeon: ni_dpm: Fix unused variable warning when CONFIG_ACPI=n

With CONFIG_ACPI=n the following build warning is seen:

drivers/gpu/drm/radeon/ni_dpm.c:3448:31: warning: unused variable 'eg_pi' [-Wunused-variable]

Move the definition of eg_pi inside the CONFIG_ACPI 'if' block.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>