drm/amd/pm/powerplay/hwmgr/ppatomctrl: Remove unused variable 'fPowerDPMx'

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:31:
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c: In function ‘atomctrl_calculate_voltage_evv_on_sclk’:
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:702:22: warning: variable ‘fPowerDPMx’ set but not used [-Wunused-but-set-variable]

Cc: Evan Quan <evan.quan@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>