drm/amd/amdgpu/amdgpu_pll: Fix kernel-doc formatting, missing and extra params

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

 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:121: warning: Function parameter or member 'freq' not described in 'amdgpu_pll_compute'
 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:121: warning: Function parameter or member 'fb_div_p' not described in 'amdgpu_pll_compute'
 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:121: warning: Function parameter or member 'frac_fb_div_p' not described in 'amdgpu_pll_compute'
 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:121: warning: Function parameter or member 'ref_div_p' not described in 'amdgpu_pll_compute'
 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:121: warning: Function parameter or member 'post_div_p' not described in 'amdgpu_pll_compute'
 drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:317: warning: Excess function parameter 'encoder' description in 'amdgpu_pll_get_shared_nondp_ppll'

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>