drm/amd/display: use a more accurate check in dm_helpers_dp_read_dpcd()

We should be checking if drm_dp_dpcd_read() returns the size that we are
asking it to read instead of just checking if it is greater than zero.
So, compare the return value of drm_dp_dpcd_read() to the requested
read size.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>