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>

drivers/gpu/drm/radeon/cypress_dpm.c

@@ -299,7 +299,9 @@ void cypress_program_response_times(struct radeon_device *rdev)
 static int cypress_pcie_performance_request(struct radeon_device *rdev,
 					    u8 perf_req, bool advertise)
 {
+#if defined(CONFIG_ACPI)
 	struct evergreen_power_info *eg_pi = evergreen_get_pi(rdev);
+#endif
 	u32 tmp;
 
 	udelay(10);