drm/msm/sde: check cached connectors for partial update caps

Use CRTC's cached connector array, which contains all connectors
attached to the CRTC regardless of the atomic delta state, to
determine whether the currently attached connector supports
partial update.

Change-Id: I2c8898b082f519189a5176e9346d86b70c2b051c
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>