drm/amd/pm/powerplay/hwmgr/ppatomctrl: Fix documentation for 'mpll_param'

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

 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:290: warning: Function parameter or member 'mpll_param' not described in 'atomctrl_get_memory_pll_dividers_si'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:290: warning: Excess function parameter 'dividers' description in 'atomctrl_get_memory_pll_dividers_si'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:339: warning: Function parameter or member 'mpll_param' not described in 'atomctrl_get_memory_pll_dividers_vi'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:339: warning: Excess function parameter 'dividers' description in 'atomctrl_get_memory_pll_dividers_vi'

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>